Point response

Quick (and non-efficient) implementation of PointResponse methods. This should pave the way for wsclean to ask EveryBeam a beam response at a facet centroid (in ra, dec).

Please check the interface carefully. In particular, do we want to pass direction as ra, dec or rather as J2000 or itrf vector?

Follow-up story will be created to implement more efficient overrides for PointResponseStationper telescope type.