Consider union of genes when merging, not intersection

[sjspielman]

In the current code to merge SCE objects, we subset all SCEs (the RNA experiments, specifically) to only include the intersection of genes in the final merged object. We may want to expand this to instead consider the union of genes. A PR for that was started here, and we'll circle back to it (there will probably be some fun code conflicts, but we'll want to keep the code and comments for future use) - #247
Direct link to branch - https://github.com/AlexsLemonade/scpcaTools/tree/sjspielman/150-union-of-features

[sjspielman]

Noting, as mentioned #247 (comment), we're very unlikely to do this ever. Keeping the issue open for now, but it may be worth just closing.