Refactor `Pauli` and related modules

[EarlMilktea]

Description of the change:

• change module structures to resolve circular import
• introduce Pauli namespace
• cleanup duplication
• add tests

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.78022% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (c038027) to head (b40b7ea). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/fundamentals.py	99.48%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #228 +/- ## ========================================== + Coverage 78.10% 78.99% +0.88% ========================================== Files 38 40 +2 Lines 6007 6113 +106 ========================================== + Hits 4692 4829 +137 + Misses 1315 1284 -31 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[shinich1]

@thierry-martinez