Major update of experimental weighting scheme (based on CASA weighting).
Merge pull request #9 from henrikju/develop

Major update of experimental weighting scheme (based on CASA weighting). Probably more work needs to be done to test the weighting scheme for correctness. There might be an issue with an inconsistency between the weighting and the gridding (concering the grid-correction), which stems from a difference between the gridding approach of pyrmsynth and the gridding as probably implemented in CASA (on which the code snippet for the weighting is based on).