Implement "seqvars prefilter" command

[holtgrewe]

Is your feature request related to a problem? Please describe.
Between seqvars ingest and seqvars query, it would be useful to have a command that downfilters the variants.

Describe the solution you'd like
Implement seqvars prefilter that takes seqvars ingest output and only keeps variants with

• population frequency
• exon distance

are below a threshold given on the command line. This allows to generate a number of prefiltered files after seqvars ingest which greatly reduce the I/O amount for queries that only consider <1% variants and <200bp exon distance anyway.

Describe alternatives you've considered
N/A

Additional context
N/A