Covering_radius method (too long docstring and mandatory guava)

The docstring of covering_radius method in linear_code.py seems too comprehensive. It could be changed to something smaller.

Beyond that, it forces the user to use Guava/Gap, which I think is bad as Guava/Gap are optional packages. We should propose an implementation that does not require Guava (and even delete this call to Guava if we manage to find something faster).