Add multi-origin SSS method to Maxwell Filtering

[xannnimal]

Reference issue #13339 Add multi-origin SSS method to Maxwell Filtering

Begin implementation of mSSS basis calculation. Added function preprocessing/fit_spheres_to_mri.py to calculate optimized spheres based on MRI, returns centers of spheres Added preliminary function in preprocessing/maxwell.py called combine_sss_basis which takes in two internal SSS matrices and combines into one internal basis.

What does this implement?

Adds functionality for using SSS with on-scalp MEG sensor systems (eg OPM)

Additional information

This is a draft and need to add functionality to accept multiple origins in maxwell.py and add functionality to calculate multiple SSS matrices using these origins, entered from user obtained from preprocessing/fit_spheres_to_mri.py or otherwise. These multiple SSS matrices can then be used in combine_sss_basis. @larsoner any suggestions/comments for implementing this part would be awesome! Thanks!

EDIT by @larsoner: Ref to https://iopscience.iop.org/article/10.1088/1361-6560/ade6ba

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