Parse write() argument.

The `write()` function argument is now parsed, so ptyx syntax is supported when
calling `write()` from a bloc of python code.

Other changes:
* randint() return a sympy Integer now.
* fix a bug when no latex file was generated.
* improve error messages.
Add options --compress, --remove-all and --quiet.

Option --compress needs a working ghoscript installation to work.