New facet grouping algorithm

Created by: nudomarinero

It should be similar to the specification of a mask filename but it does not require pyrap and I could make it work with the mask files generated by the facet generation routine of Reinout's pipeline.

It should not break backwards compatibility unless there was a mask file called "facet". It can be improved in the future to cover more user cases. At the moment, for example, it sets a value of "Facet_0" by default to the points out of the mask.