[REF] Implement _matmat instead of _matvec

[f-dangel]

Warning: This is a big refactoring and it will require some iterations to reduce complexity.

Please complain about anything that is incomprehensible, and check for outdated/unclear docstrings.

Currently, all linear operators implement how to multiply with a single vector (_matvec).
Multiplying onto more than one vector is simply iterating over the vectors one by one (_matmat).
This adds a lot of sweeps through a data set which can be avoided by implementing matrix-matrix,
rather than matrix-vector multiplication.

This PR changes the base class implementation from _matvec to _matmat.

[coveralls]

Pull Request Test Coverage Report for Build 7818184673

• -4 of 129 (96.9%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 89.077%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
curvlinops/examples/utils.py	0	1	0.0%
curvlinops/fisher.py	3	6	50.0%

Files with Coverage Reduction	New Missed Lines	%
curvlinops/fisher.py	1	30.19%

Totals
Change from base Build 7806818295:	0.5%
Covered Lines:	946
Relevant Lines:	1062

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💛 - Coveralls

[f-dangel]

@runame let's first merge #69. I will then refactor KFACInverseLinearOperator in this PR, too.

[f-dangel]

Ready for review.

[runame]

I might be missing something because I'm not that familiar with the code for the non-KFAC linear operators, but everything looks good to me and the tests are all passing.