Create schema-level constraint for start/stop on SequenceLocation within Adjacencies

[Mrinal-Thomas-Epic]

Currently, the SequenceLocation within an Adjacency is expected to have either start or stop populated, but not both. The constraint that both should not be populated is not specified anywhere in the schema, which could lead to confusion (e.g., discussion #471).

[ahwagner]

@Mrinal-Thomas-Epic this is great, thanks. Would you be interested in drafting the schema change?

[Mrinal-Thomas-Epic]

I'll take a stab at draft the schema change. Before I write it out, do you or @d-cameron have any thoughts on whether it should just be implemented as just a constraint on the use of SequenceLocation within Adjacency, or whether we should make a different class for a point + direction on a sequence?

[ahwagner]

@Mrinal-Thomas-Epic I think a constraint definition makes the most sense for forward compatibility.