Add implementation of kernel ECORR

[jeremy-baier]

Kernel ecorr should be faster than GP ecorr, which is the current implementation.
I think appendix C here has a kernel ecorr implementation: https://arxiv.org/html/2412.15858v1

[jeremy-baier]

@vhaasteren might have details on a faster implementation

[abhisrkckl]

This is a great idea. If we can implement fastshermanmorrison, it will speed up PINT fitting greatly.