Matrixkernel convenience functions and related performance improvements

[Crown421]

Summary Following up from #354 , this pull request contains the matrixkernel convenience function, some additional tests, a small fix, and performance improvements that came up along the way.

Proposed changes

• Adding matrixkernel
• Include changes discussed in #354
• Fix for helper method in fbm kernel
• Performance improvement for lmm kernelmatrix
• Fix specialized implementation for slfm to work with both MOInput types
• Related tests

What alternatives have you considered?

[Crown421]

As suggested, I am splitting this again into a few smaller PRs, not in the least because in doing so smaller bits spawned more larger issues.