Bug-Fix: MatZq::inverse_prime

[jnsiemer]

Description

This PR...

• fixes the implementation of MatZq::inverse_prime and adds tests to avoid this bug
• adds a benchmarks for MatZq::inverse,
• avoids warnings for benchmarking by commenting out a benchmarking section.

Checklist:

• I have performed a self-review of my own code
  • The code provides good readability and maintainability s.t. it fulfills best practices like talking code, modularity, ...
    • The chosen implementation is not more complex than it has to be
  • My code should work as intended and no side effects occur (e.g. memory leaks)
  • The doc comments fit our style guide