AG-1147/AG-1148/AG-1098: Add tests for rna seq data and rna distribution data

[jaclynbeck-sage]

This PR includes changes for 3 interrelated tickets:

AG-1098: The rna_distribution.py transform had 2 transforms in it (transform_rna_seq_data and transform_rna_distribution_data). The first transform is used both on its own and as a pre-requisite for the second transform. The first transform has been moved to its own file. Additionally, I renamed it (transform_rna_seq_data -> transform_rnaseq_differential_expression) to match the name that is in the config.yaml file.

AG-1147: Added a data-driven test for transform_rnaseq_differential_expression. There is a "good" input file and an input file with some missing data in each of the important columns, which should both pass. There is also a "bad" input file, where the logfc field is a string instead of a number, and this is an expected failure case.

AG-1148: Added a data-driven test for transform_rna_distribution_data. There is a "good" input file and an input file with some missing data in each of the important columns, which should both pass. There are TWO failure cases:

1. The logfc field is a string instead of a number, which throws an error while calling transform_rnaseq_differential_expression
2. All model entries are blank or all tissue entries are blank, which causes all rows to be dropped while calculating quartiles, and throws an error.

I made sure that both tests pass post-refactor, and that the transformed output on the real data is identical to the previous version.

[JessterB]

LGTM!