Stats about blind spots (N) when matching genomic profiles

[matthuska]

It would be nice to be able to differentiate between three states for each position in the genome:

1. matches the reference
2. does not match the reference and is some other nucleotide or a gap
3. and unknown (=N)

That third case isn't handled well by covsonar, so it would be nice to have some idea when you query for a position, how many sequences have an unknown (=N) at that position. Maybe this information would be included using an optional argument.