Fix recomputing ncp ncp0

The current implementation force the re-computation of the north celestial pole for each station in a phased array. Recomputing the north celestial pole seems to be a not to expensive operation, however, it result in many calculation of the transformation matrices in casacore::measures. Such matrices as mainly dependent of time so the result can be cached.

In this merge request the interface of everybeam is adapted to be able to cache the NCP computations.