Fix code-example syntax in documentation

The trailing ``::`` mean that the string ``:file:`volume.pxd`::`` was being treated as the code example and therefore ``cdef float cube(float)`` was simply indented as regular font.

"nogil" for C++ casts

add IPython notebook screenshot to the docs

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extra : transplant_source : -%C1%5B%21Igwr%87%0A%03%E7%D4%E8%A7%19%97%F1%D8p

avoid LLVM warning: skip ternary operator when value is unsigned

--HG--
extra : transplant_source : %87%E8%C9%0A%B8%1A%C34%26H%B8%0F%89%D4%8CC%10%1A%0F%84