Consider union of genes when merging, not intersection

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

Start by reviewing the current SCE merge implementation and the work on branch sjspielman/150-union-of-features, along with issue #247. Compare the existing intersection behavior with the proposed union behavior for RNA experiments, and preserve useful code and comments from the earlier work where appropriate. Done means merged SCE objects retain the union of genes.

Written by the indexing model from the issue text.