Alphabet as inherent property of a sequence collection

Related issues: #16, #8 (specifically https://github.com/ga4gh/seqcol-spec/issues/8#issuecomment-815926785).

This came up during the Sequence Collections call at Connect. I think that this is an important feature that would help us resolve a generalizable issue with the interpretation of sequences that is backwards compatible with refget.

Since @andrewyatz was kind enough to raise this on the call I thought I would create an issue here to track discussion on this specific feature request.

@sveinugu @nsheff

Start by reading related issues #16 and #8, especially the linked comment, then review the discussion referenced from the Sequence Collections call. Clarify the proposed inherent alphabet property, its backwards-compatibility requirements with refget, and the specification changes needed before implementation can be considered complete.

Written by the indexing model from the issue text.