Add required memory functions for non-gradient boosted equivalent sources

[mdtanker]

I find I commonly want to check whether I can fit a jacobian in memory for regular (non-gradient boosted) equivalent sources, but have no easy way to check whether this is true, of if I need to switch to gradient boosted. This PR adds equivalent estimate_required_memory functions for cartesian and spherical equivalent sources.

[santisoler]

It looks good to me, @mdtanker! Thanks for adding this, I get how helpful it can be.

I left a few small suggestions to the docs, feel free to check them out.

Also, I fixed tests failing in Harmonica, so if you update the branch, your tests should pass now.

Thanks again!