Use "matched_coords" pr PRISM file and add prsn to computable indices

[eyvorchuk]

Originally, the pr and tasmax grids for the 1981-2010 PRISM data differed by about 5e-11 degrees, so finch.prsn could not be computed. Stephen put together a pr file with the same grid as the tasmax one under /storage/data/climate/PRISM/dataportal/pr_monClim_PRISM_historical_run1_198101-201012_matched_coords.nc, which should be used for pr computations in this tool.

Additionally, the prsn index with the auer method should be added to the multivariate indices. Note that this is only computable at the daily resolution instead of other time resolutions like the other indices, so the widget and parameter setup for this index will have to account for that.