enable multiprocessing for spectral, wavelet and cross-wavelet analysis

[khider]

Enable multiprocessing for the significance test. Speed up is about x6 on Mac M3.

1. Functions that were added at the top of modules are helper function that NEED to be outside a class to function. They have an _ at the beginning so they are set to private and won't appear anywhere.
2. The Series class needs to be imported inside the functions for coherence for pickling issues. Do not remove the line
3. For coherence, dill was necessary (Series is not pickable apparently).
4. General clean up to the code base since I was at it.

[CommonClimate]

I trust that this all works!

Looks like several new packages are being used, but I don't see changes to environment.yml and setup.py. Please update to make sure it plays nice with the rest of the package.

[khider]

I added dill (only package that is not from Python itself) and tried to fix some of the docs issues as well.

[CommonClimate]

Green light!