updates to make cellbrowser.py faster and easier to use

[swemeshy]

March 6:

• split _make_cell_browser_files into multiple functions so files can be updated individually if needed
• create exprMatrix.tsv.gz instead of expr_mtx.csv.gz so UCSC cellbrowser module copies the file instead of rewriting it (this makes it slightly faster)
• some general updates for the latest version of UCSC cellbrowser package

March 10:

• used pd.DataFrame.sparse.from_spmatrix when creating dataframe for making exprMatrix.tsv.gz, which uses much less memory