Retrofit multi-species annotations to fit new annotation system

[dosumis]

Retrofitting all use of 'host cell X' terms is required in order to obsolete these terms as part of re-arrangement of the extracellular component branch. This requires a mapping between all host cell X terms regular CC terms; a default retrofit spec for all combinations of organism where relationship is always the same (e.g. virus -> anything else); a default retrofit for where only one taxon is mentioned in the existing annotation (use 'host organism' from PCO).

Retrofitting other multi-species annotations (some CC and all BP) is not essential, but we should at least provide a guide.

(Issue moved from JIRA)

[dosumis]

This can be done in phases. We can freeze addition of new terms to 'host cell X' branch while retrofit is underway. Only obsolete after.