Bugfix: SNP records with N as ALT now noted as SNPs.

The VCF 4.0 and newer specifications say the ALT field is a comma
separated list that includes "base Strings made up of the bases
A,C,G,T,N". Notably, the last case was not handled by `Record.is_snp`,
causing it to erroneously report `False` for records with "N" as the ALT.