/reviewed-on !269

  1. 'Solve Divergences' action:
     => Display Delivery_viewSolveDivergenceDialog.listbox:
        -> Delivery_getSolverDecisionList
           -> Delivery_getSolverProcess => create Solver Process in 'draft' state and build Solver Decisions.
  2. Clicking 'Update' button updates Solver Decisions.
  3. Click 'Solve Divergences' button after either 'Adopt Prevision' or 'Accept Decision'.
     => SolverProcess.solve():
        Solver Process transit to 'solving' state and 'solve' Activity is created.

Before this Activity is executed, user can still select 'Solve Divergences'
action and this will create another 'Solver Process' as the current one is not
in 'draft' state anymore. Moreover, another user can still 'Adopt Prevision'
(while the first user selected 'Accept Decision') and this will be actually be
applied despite the first user choice.

Instead, create Solver Process when transiting to 'diverged' state and do not
display 'Solve Divergences' button nor allow access to 'Solve Divergences
Dialog' (Delivery_viewSolveDivergenceDialog) if there is a Solver Process in
'solving' state.

erp5_simulation: Display Divergence dialog even if there is nothing to solve (same as legacy Simulation Dialog).

This fixes the following Site Error when there is no Divergence to solve:
  AttributeError: 'NoneType' object has no attribute 'SolverProcess_getSolverDecisionList'

This may happen if user selects 'Solve Divergence' Action and solve
Divergences, then immediately goes back and selects again this action
whereas there is actually no Divergence to solve.