On the use of Guava

Some methods in linear_code.py require Guava, an optional package of Gap to work. As this package is optional, all these methods should also propose a Python alternative algorithm or at least check is Guava is installed and fail with a proper error message if not.

Besides, it could be nice to see if a Python implementation is faster than the Guava one in these methods, and if it's the case, Guava call should be removed.