Filter out sequences that contain unknown amino acids

https://jira.gsc.wustl.edu/browse/AT-623
If this will be called a lot, it might be better to create the regular expression once, something like:

```perl
sub _generate_valid_sequence_regex {
    my %conversions = %Genome::Info::CodonToAminoAcid::convert;
    delete $conversions{Z};
    my $allowed_amino_acids = join('', keys %conversions);
    return qr/^[$allowed_amino_acids]+$/;
}

sub is_valid_sequence {
    my $seq_string = shift;
    state $valid_sequence_regex = _generate_valid_sequence_regex;

    return $seq_string =~ $valid_sequence_regex;
}
```