genes.tsv not downloaded if dataset is specified in cd.download function

[ymahlich]

If specifying a cancer dataset in cd.download() (and on the CLI via coderdata download --name <DATASET_NAME>), the genes.tsv table is not retrieved from figshare.
Only when downloading all coderdata datasets (name=='all') genes.tsv is retrieved.

The retrieval of genes.tsv must be added into the download routine independent of whether a dataset is specified.