Genome insertions

[jakebeal]

We should allow strain designs to be specified by using a strain identifier (e.g., E. coli DH5-alpha) and a locus designation for the planned insertion point.

Strain identifiers need to be URIs, but would likely be external references, e.g., into NCBI RefSeq or txID (for example, E. coli DH5-alpha is NCBI:txid668369). This will typically be included as ExternalReference objects, rather than being fully imported.