rate.py:  simplify Bins' handling of slices, update some doc strings
pylal.rate.Bins.randcoord:  tweak handling of infinite bins

- infinite bins are not sampled from, but we weren't correctly adjusting
  the PDF to reflect this.  this patch changes how infinite bins get
  skipped so that the PDF correctly relfects that they are not included
- add examples designed to test the normalization of the PDF under various
  constraining conditions
rate.py:  add example to NDBins.randcoord()

- thereby subjecting this method to a test of its operation.
- also edit a variable name
rate.py:  more expansion of docs/tests and some aesthetic changes