Enable skipping comment lines while sniffing files #4239
This is related to #3148.
For some data types, sniffing is done after skipping comment lines. That's problematic in cases of files with many comment lines at the beginning, where false assignments can be done (as in #3148). This PR adds a
As an aside, this causes the most recent dm6 GFF to sniff as GFF rather than BED :)