Add two-bit encoding for generate ancestors

[jeromekelleher]

Following up on #809, add a two bit genotype encoding that'll support missing data and three alleles.

Probably not a priority for the moment since most (phased) datasets that are sufficiently large to need this seem to be strictly biallelic.

[benjeffery]

Great, note that UKB has a significant fraction of tri-or-more sites. Our current plan was just to filter them.

[hyanwong]

Also there was some talk about re-imposing the missing sites over the top of the phased datasets. But that's waay down the line.

[jeromekelleher]

I'm not sure the ancestor generator supports multi allelic anyway, so I guess it's just missing data we need to consider for now

[benjeffery]

Yep, we have zero missingness in both datasets.