inspiral_lr.py: Add population model to ranking statistic. Population model information in inspiral_intrinsics.py.

- move P_instruments_given_signal() and joint_pdf_of_snrs() from far.py to
  new module

- move to separate module in gstlal.stats name space
- fix "bounds out of order" with open-ended slices: when open-ended slice is beyond end of the keys currently in the tree
- also make a few adjustments:  use isinstance() to check for slices in NearestLeafTree, check for negative horizon distances in quantize_horizon_distances()
- add .weighted_mean() methods

- move some common general statistics routines from gstlal-inspiral's far.py into the gstlal.stats.__init__ module