Variables are investigated, recursively in case of container objects (like lists, for example),
to detect if they can be stored in the ZODB.
In this investigation persistent objects are identified by being an instance of the object
class and implementing a `__getstate__` method that raises no exception. If the variable is
not a Persistent object then we try to pickle and load it.

While developing the pickleable object identification a complication was found. It seems that
the code cannot capture cPickle.PicklingError in the usual way, `except cPickle.PicklingError`.
It's consequence of some weirdness with regards to pickle/cPickle modules exceptions classes and
more about it can be read at http://bugs.python.org/issue1457119. So, the workaround for this complication
was to catch all exceptions and check the exception class name as string.

The whole check for zodb persistence was moved into an utility function for the sake of readability
and code maintenance.

The Base_executeJupyter script object was transformed into an extension to be able to properly handle
transaction errors and render them correctly inside Jupyter.

erp5_data_notebook: moves variables and setup storage from ActiveProcess to `notebook_context` property

Notebook's variables and setup were being stored in an ActiveProcess, which is
not needed anymore, because now everything related to user context can be
safely stored in the ZODB.
They are stored in a `notebook_context` property of the Data Notebook
itself. Code and tests were updated properly. The old `process` property was removed.

All the references to *_variable_dict were renamed to  *_notebook_context, documentation
and tests were updated. Related objects like scripts and external methods were renamed
too.

To store objects in the `notebook_context` property we do 2 different tests: the first
to check if the object can be serialized by `ZODB.serialize.ObjectWriter`. If the first
test fails we test if the object can be serialized by cPickle. For the second test we
need to dump & load the object to be completely sure that it can be correctly loaded
later.

The function called by the Base_runJupyterCode external method was renamed from
Base_compileJupyterCode to Base_runJupyterCode be more consistent and avoid confusion.

All errors while running setup functions and now properly propagated to the user
interface in Jupyter and code execution is aborted.

- An environment object was implemented to help us deal with the multiprocess
architecture of ERP5 and objects that cannot be easily stored in the ZODB.
It stores definition of functions, classes and variables as string. The implementation
uses a dumb Environment class to allow users to make `define` and `undefine` calls,
which are captured and processed by an AST transformer before code execution.

- Along with the environment object, an automatic "import fixer" was created. It does
not allow users to import modules as they normally would, because this may cause
collateral effects on other users' code. A good example is the plot settings in the
matplotlib module. It will fix normal imports, make them use the environment object
mentione earlier automatically and warn the user about it.

A few bugs were fixed with this implementation, for example:

- https://nexedi.erp5.net/bug_module/20160318-7098DD, which reports an inconsistency
on portal catalog queries between Jupyter and Python (Script) objects. Probably an
issue with user context storage in ActiveProcess

- https://nexedi.erp5.net/bug_module/20160330-13AA193, which reports an error related
to acquisition when trying to plot images, which happened in other situations, although
this is not officially reported in Nexedi's ERP5. This probably also was happening because
of old user context storage.

- normalize path to avoid to traverse with wrong value
- review KeyError message when domain is unknown
- use getWebSiteValue when possible

/reviewed-on !124

Dependency should be defined directly on bt5 if needed.

'keep_existing' argument of Base._edit has never been used
and we should consider removing it.

When defining a guard on an object, calling
a method such as Base_viewDict on that object
could be denied due to guard acquisition.

- the check is moved to a less performance-critical place
- getPortalObject was forgotten in commit d8e9ed19

- 'apply' causes a break in cProfile results
- 'apply' is deprecated (removed in Python 3)

* reference
* translated_title
* the usual owner_title, creation_date and modification_date
* remove useless listbox_editable_fields that are defined in the field library

This business template does not seem to follow the guideline yet

For example, Python Scripts were:
- dirty in the imported BT (causing all of them to be reinstalled on upgrade)
- compiled to return None

When a currency exchange line is (in)validated, we need to reindex the currency exchange
cells that acquire validation state from the currency exchange line.

This allows defining an exchange rate without end date

https://codemirror.net/demo/activeline.html