Several tasks in calibration product production (and some others actually) will work on visit pairs rather than single visits.

There is already an implementation of a custom TaskRunner in cp_pipe/makeBrighterFatterKernel.py:MakeBrighterFatterKernelTaskRunner. This should be made into a utility in cp_pipe so that its other tasks can use it, rather than either reinventing the wheel, or importing something misnamed and in the wrong place.

1) Move that task runner out of makeBrighterFatterKernel.py
2) Get makeBrighterFatterKernelTask to work using the new runner.