Add getstate and setstate for PolychromaticBeam.

[toastisme]

Override setstate and getstate methods for PolychromaticBeam to fix error when using copy.deepcopy.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 42.20%. Comparing base (080b599) to head (2ff70b4). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #711 +/- ## ======================================= Coverage 42.20% 42.20% ======================================= Files 187 187 Lines 16743 16743 Branches 3191 3191 ======================================= Hits 7067 7067 Misses 9031 9031 Partials 645 645 ```