snappy pull-processed-data: accept list of file type as argument

[eudesbarbosa]

Issue
The current implementation of snappy pull-processed-data takes only one file type as argument per execution (arg: --file-type). That is slightly inconvenient for when one wants to download several file types. For example, for CADS execution both VCFs and BAM are required, so the command is called twice:

cubi-tk snappy pull-processed-data --file-type vcf ...
cubi-tk snappy pull-processed-data --file-type bam ...

Possible Solution
Change argument --file-type from string to list. Change the impacted methods accordingly.

cubi-tk snappy pull-processed-data --file-type bam,vcf ...

Alternatives
None.

Additional context
None.