(Code clean-up) Remove all commented code.
Update sympy.

Small changes were made in mathlib (a new bug was found in sympy: issue 3588),
to cope with new sympy version.
LaTeX generation is slightly improved.
Improve Tablatex user interface.

Selecting an option will automatically update generated code, without
having to press [Enter].
Improve sessions handling and concurrent access to session file (autosaving).

Each session has now an unique ID. Previous sessions can be easily reloaded.
(As much as 20 previous sessions are saved by default).
Fix small issue in printing of functions.

In functions involving 'ln', 'log' was printed instead.
Add `approche` keyword to tabvar(), to return rounded values.

(This is needed by Ptyx project).
Minor doc update.
Minor changes.

* Don't diplay splashscreen if initialisation.py is imported as a library.
  This is mostly used in tests.
* Add session number to data collected by `infos.py`.
[Interpreter] Use Decim and not Float internally.

Decim is a subclass of Rational, so all computation is done with rationals
internally, but Decim are displayed as decimals.

Using rationals internally avoid approximation problems ; most of the time
decimal display is preserved, but this can't be taken for granted.