As part of RFC-183 document the fact that variant tasks should have a common abstract base class that defines the API. If we add future tasks that we feel are likely to have variants, then we should create an abstract base class.

Candidates include star selectors, PSF determiners and ISR tasks.

Note that this applies to tasks LSST provides in its stack, not to variants users produce and other obscure one-off code.

Also document the desire that tasks with anticipated many variants, such as star selectors, and PSF determiners should be in registries. This explicitly excludes tasks such as ISR where only one task is likely to be useful for a given set of data.