Fix early return and counting of mismatches with no-calls in barcodes.

[theJasonFan]

Closes #47.

The snippet from barcode matcher was returning None too early if there are Ns in both expected barcodes and observed barcodes.

fn assign( ...) -> ... {
...
    if num_no_calls > self.max_mismatches as usize {
        None
    }
...

E.g. When max_mistmatches == 0, observed barcode ANAAAA should match NNAAAA, but the snippet below returns None early.

To allow for no-calls in barcodes, we have to relax the optimization here and exit early if the number of observed no-calls is greater than the maximum number Ns for any sample barcode plus the maximum allowable number of mismatches.

[nh13]

Really nice catch!