This adds the flag `GATEWAY_ENABLE_GENE_SETS` to enable support for gene
sets. To simplify implementation, activating this flag also activates
`GATEWAY_ENABLE_ANNOTATIONS`. The gene sets are saved in a file that
has the same name as the annotations `csv` but with `_gene_sets`
appended to the file name (before the extension). This file is hidden
in filecrawler, and the gene sets are loaded when the associated
annotations file is loaded.
If the annotations file is missing, then an Exception is raised.
I have updated one unit test to make it expect
`--disable-gene-sets-save` in the default case (i.e. if
`GATEWAY_ENABLE_ANNOTATIONS = 0`). All units tests pass.
I have updated the README to document `GATEWAY_ENABLE_GENE_SETS`.
