move SequenceVariant.fill_ref() to separate module

[reece]

SequenceVariants have a fill_ref method that requires a data provider instance. This violates the intention to have the classes be data models with intrinsic methods only (i.e., methods that operate only on the data they have). More importantly, it creates circular object dependencies that are, um, awkward.

[github-actions[bot]]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[github-actions[bot]]

This issue was closed because it has been stalled for 7 days with no activity.

[reece]

This issue was closed by stalebot. It has been reopened to give more time for community review. See biocommons coding guidelines for stale issue and pull request policies. This resurrection is expected to be a one-time event.