Add a custom round() function.
Improve randfrac() and freeze random generator state when using #TABVAR...

* Add ability to choose denominator for randfrac() and srandfrac().
* Save random generator state before using #TABVAR or #TABVAL,
  and restore after.
  This is because sympy.solve() may modify it as a side effect.
* Add several failing tests (bugs to be investigated later).
Improve #SEED tag behaviour for multicompilation.

For each compilation, SEED value is reset and then incremented according
to compilation number.
Minor test change.
Add min() and max() to global_context.
Improve #* tag.
Add randbool().
Improve varname detection and fix decimal separator issue.
Add ['floats'] evaluation mode. Implement pdf page reordering for printing.

To avoid confusion, previous ['float'] mode was renamed as ['.'].

To reordering are defined for now.
Replace rationals_to_floats() by more efficient numbers_to_floats().

Other improvements:
- Add #GCALC tag to execute a command in geophar Interpreter.
- All sympy variables and functions are now imported inside
  `global_context` name space at startup.
- Fix a bug involving name variable.