#1 Rewrite parser

[Renaud Métrich]

The parser should be rewritten in order to conform to
the Math writing standards (LaTeX standard). This means:
- variables and functions identifiers should be in Italic
- known functions (cos, sin, tan, ...) are in Normal face
- numbers should be in Normal face
- if 2 terms contain numbers and vars (ex: 2x and 4y),
then an implicit product of these should have a space
between the terms:
- ex: 2x * 4y -> 2x 4y
- ex: x * y -> xy
- general rule for A * B
- A = number -> no implicit product
- B = number -> no implicit product
- B = number and var -> implicit product with a space
- B = var -> implicit product with no space