streamline parseVEP for multiple files

[lydiayliu]

Currently, --vep-txt in parseVEP allows the input of multiple VEP text files, and parseVEP parses them together to create a single tsv file for later peptide database creation.

I have a different use case, for example I have to parse germline indel VEPs for 73 samples, for every sample VEP takes 1 minute to read the index (which is already really fast) but takes no time at all to parse the files (literally zero seconds). Could there be an option to input multiple files but not combine them? That way the index only needs to be loaded once and it would probably be 100x faster.

Just a thought, we can discuss if this is necessary.