Option to ignore specific fields (NNNN, UUUU)

Allow an option to ignore alleles with certain suffixes (e.g. `NNNN,` `UUUU`)

It's helpful when trying to reduce GL String which may contain alleles that signify absence e.g. `DRBX*NNNN` in
```
DRB3*03:01+DRBX*NNNN
```