add ensemble selection function

[cmahony]

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

[kdaust]

• Add KKZ function to climr
• Add option to downscale where gcms/runs can be passed it vector form (e.g., gcms = c("ACCESS","ACCESS","EC-EARTH3"), run_nm = c(1,2,1)