Remove GriddedResidualConnector from the codebase

[iancze]

As described in #154 and related issues in #126 , #110 and #100, the GriddedResidualConnector class will be largely redundant once the Briggs weighting residual imaging process (#110) is fully integrated into the codebase.

We should remove GriddedResidualConnector from the codebase entirely and update all code that uses it to follow the Briggs/NuFFT residual imaging process. Once this is done, the only remaining routine in connectors.py will be index_vis, which should probably be moved to datasets.py.