Numerous small fixes in `mathlib.Interprete`.

 * convert internally decimals to rationals to improve behaviour.
 * remove old unused options.
 * add tests.
 * update slightly str and latex printers.

FIXME: `frac()` doesn't work anymore.
Fix regression caused by previous commit.

Also add more tests and fixe an issue with '0,3[17]' notation in calc.
(Solver) Convert floats to rationals before solving.

This improve gratly solving for rationals.
Also fix a small regression concerning floats (adding infinity and a float).
Add support for pyflakes format in `tools/edit.py`.
(Refactoring) Create `mathlib/printers.py` file (latex and str printing).
(Refactoring) Remove unused internal function in mathlib.
(Refactoring) Create `mathlib/solvers.py` file for (in)equations solving.
Fix bug in last level of 'exercice_equations_droites'.

Last question was not displayed, and sytem was not explicitly given.
Add new module 'Bienvenue'.

This module provides a simple  presentation of geophar.
Make new 'reload.png' icon.
New session management interface.

Also set 'param.ligne_commande' to 'False' by default, since this is used
only by advanced users.
Add pylint config file (needs to be tweaked).
[wx->Qt] Fix autosave.

This should end transition from wx to Qt !