gstlal-inspiral: split off horizon distance history

- 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