Refactor and expand onepass model

[sjfleming]

Closes #163
Closes #296

This is a refactor of onepass to make it more extensible. It implements the Welford algorithm for online variance calculation and it also implements a gene-gene covariance computation via an online algorithm similar to Welford (https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Covariance subsection "Online"). The latter can be run using the raw data or ranks (which allows for a computation of gene-gene Spearman correlations).

Currently the Welford implementation is actually implemented in a different cellarium class, as is the covariance implementation. I thought this might be cleaner than having one huge class with more input arguments, but I'm open to opposing views. Also, the class heirarchy worked a lot better when Welford was a separate class, since Welford keeps track of different sufficient statistics than the naive/shifted algorithms. And the Welford-like gene-gene covariance keeps track of the same sufficient statistics as Welford (plus more).

Todo:

• the refactor of the existing naive and shifted algorithms must pass existing tests
• add tests for the Welford variance calculation
• add tests for the Welford-like covariance calculation
• add tests for the Welford-like correlation calculation
• add tests for the Welford-like Spearman correlation calculation
• ensure Welford works on more than one device
• create CLI for Welford
• create CLI for Welford-like covariance
• add CLI tests for Welford
• add CLI tests for Welford-like covariance

[sjfleming]

Welford covariance computation on all 33k genes (using a batch size of 10k cells) spikes to like 31GB memory usage on my laptop (eyeballing Activity Monitor). Just wanted to make a note of this as a ballpark figure. I am guessing that's why the github actions runner went OOM on 5495ce0. The CLI test now uses a Filter and only computes covariance using a handful of genes.

For speed purposes, the rule seems to be "make the batch as big as you can accommodate in memory".

[sjfleming]

Ensure batch is not empty in update()