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'.

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