Metacal catalog class

[NiallMac]

I've been thinking for a while it would be useful to have a metacal catalog class in Treecorr that would provided a standard/well-tested piece of code for calculating and applying the shear calibration corrections to two-point functions. From DES experience, it seems this part of the pipeline is pretty prone to subtle user errors, this may be a good way to address that. @rmjarvis - @seccolf and I would be happy to work on this if you think this is a good idea.

[rmjarvis]

Sure. At the very least, putting an example script of how to do it in the TreeCorr repo would probably be useful. But if you see aspects of that, which would benefit from having treecorr.something functions to assist, please do so.