Proposal: Function for optimized retrieval of GT from FORMAT field

[cmdcolin]

fully parsing out the "FORMAT" field requires a lot of complex memory structures (Object containing Object containing arrays of strings or numbers or ...). this can cause out of memory and lots of garbage collection during parsing large 1000 genomes type data.

this PR proposes a simplified representation called parseGenotypesOptimized

this PR is connected to efforts here, which parses large regions of 1000 genomes type data GMOD/jbrowse-components#4511

It also makes

• changes SAMPLES to a function instead of a Object.defineProperty. This makes it more explicit that it is a lazy getter operation, and to me makes the code a bit simpler
• adds a GENOTYPES function call that only parses out the genotypes from the FORMAT field
• Improved typescript as a result of this

Footnote: overrides #94 probably. In #94, I was very committed to preserving the notion of what the existing Variant class was, but this PR changes it. as a result, it's a major version bump.

fixed #98