Add parallel TSC module

[lgarrison]

This adds a multi-threaded TSC module under abacusnbody.analysis.tsc, as a first step to improving the performance of the power spectrum module (#70). The basic algorithm is to partition the particles into stripes and parallelize the TSC over stripes. I get really good performance, over 500 M particles/sec with 32 cores on a small grid (but this mostly depends on the memory bandwidth of the platform).

I also moved the power spectrum and 2PCF files out of the hod package and into abacusnbody.analysis.

I added linting using ruff, so there's a number of small stylistic changes to some of the files that were moved (because ruff runs against any files that were touched in the commit). The most common non-trivial issue flagged by ruff was unused variables; I commented those out in the source code in case they were there for some (future) reason.

@boryanah, @SandyYuan for visibility

[lgarrison]

@boryanah do you want to take a look at this? I wrote an overview of the new TSC module here: https://abacusutils--79.org.readthedocs.build/en/79/tutorials/analysis/tsc.html . Let me know if you have any suggestions before I merge. I'll look at the power spectrum part of the code after this.