pylal.rate:  move .__getitem__() slice handling to Bins class

- also fix off-by-one bug in the returned upper bound
- and add examples to doc strings to cover slice handling with unit tests
rate.py: .randcentre() --> .randcoord()

- instead of returning randomly chosen bin centres, return randomly chosen
  continuous coordinates
- also allow the domain over which the values is chosen to be controlled
cbcBayesPostProc.py: Fix email option
LALInferenceProposal.c: Disable sky ring proposal and adaptive proposal for Nest as these break detailed balance