Allow users to link multiple evidence methods to a single annotation

[lreiser]

This function would allow a user to associate multiple evidence methods to a single gene-term-publication annotation.
The use cases are

1. Cases where similar types of experiments (e.g. binding assays) are performed that reinforce the association. For example: protein interactions that are based on a combination of yeast two hybrid, plus BiFC, plus Co-immunoprecipitation.

2. Cases where different experimental results yield a synthesis (often biological process) annotation. For example, a combination of enzyme assay plus overexpression data plus RNAseq could yield an annotation to XX homeostasis. That is different lines of evidence, when combined, support an assignment. NB: for curators- to check how GO is modeling or thinks about modeling these types of annotations.