Reword sequence-level definitions

[nataled]

Current: "a translation product of some mRNA giving rise to a protein with the amino acid sequence represented by UniProtKB:"

Proposed: "a translation product of some mRNA whose exon structure is the same as that which gives rise to a protein with the amino acid sequence represented by UniProtKB:"

This makes it clearer that we are not talking about a specific protein sequence, and thus allows variants to be classed with like isoforms, and justifies the orthoisoform concept.

NOTE: this will require revision of a number of scripts that depend on this wording.