F/103 posterior allele frequency

[timothymillar]

Fixes #103

• Addition of the AFP field(s) for both sample and population posterior allele frequencies.
• Note that this doesn't add posterior dosage yet because there are some questions that need to be resolved there.
• This also improves performance of calculating posterior allele frequencies and posterior allele occurrence.
• Also updates the help text in the repo which was missed in an earlier PR.