Added the option to supply multiple mask files to VariantFiltration

[KevinCLydon]

I changed the --mask and -mask-name arguments to be lists so it's possible to supply multiple mask files. There are still some questions to discuss that may warrant changes:

1. Should -filter-not-in-mask also be a list, so the user specifies whether to do a mask or reverse mask for each file?
   a. My inclination is no, since that would make things kind of complicated and probably you just want to filter variants that appear in none of the mask files
2. What should maskName default to now?
   a. Previously, it defaulted to "Mask".
   b. I changed it to default to "Mask" for the first mask, and then "Mask2", "Mask3", etc. Not sure if this is ideal?
3. Should the variable names be changed?
   a. i.e. mask -> masks and maskName -> maskNames
   b. Obviously the arguments would keep the same names
4. When using -filter-not-in-mask, what should we list for filters?
   a. All the mask names? (this is what I'm doing now, but it could obviously get very long and maybe be misleading?)
   b. Should we just allow one -maskName if -filter-not-in-mask is specified?
5. Is my implementation likely to cause a prohibitive performance reduction?

Closes #8119