Limitations of vtools genotype database #11

[BoPeng]

Moved from https://github.com/vatlab/vat-dp/issues/11

1. It is not sparse for simple scenarios (where genotypes are coded 0/1/2/NA)
2. It does not distinguish between 0 and NA, unless we specifically asks it
3. It does not store phase information
4. It only accepts integers, not imputed genotypes
5. Storing various annotations makes the genotype database painfully complex