Implement sparse genotype as linear operator

[quattro]

Catch-all for implementing centered/standardized genotypes as a sparse linear operator

• [x] JaxSparse implementation of genotype with jax.Array backends for dense covariates, mean-terms, scaling, etc...
• [x] Lineax implementation of linear operator to represent the centered/standardized genotype data
• [x] unit testing of genotype / vector and genotype / matrix operations

[quattro]

This is more or less done, but limited to the BCOO datatype in JaxSparse. Leaving this open for now, if we can extend to BCSR at some point.

[quattro]

Another enhancement would be a "blocked" or "batched" version of this that captures different genotype matrices over different chromosomes. This would have a benefit for downstream solvers by treating the large covariance matrix as block diagonal.