add ensemble selection function

climr needs a function that selects ensembles of GCMs and runs.

- [ ] select small gcm ensembles based on KKZ and other criteria
- [ ] select a subset of runs that represent the larger ensemble of runs, based on KKZ

The user would supply a point, polygon, or bbox. the function would create a small number of points (as few as ~8) representing the spatial range of the bbox, and select the ensemble based on these points