functionality for user-provided reference climatology

a user may want to downscale to their own reference climatologies, instead of the PRISM climatologies provided by climr. it would be nice if they could pass this local object to the climr_downscale() function (e.g., as the "which_normal" variable).
one thing to consider here is how to handle a different reference period (e.g. 1981-2010).

Start by reading the climr_downscale() entry point and how it currently selects the PRISM climatologies through which_normal. Define how a user-provided local object and a different reference period such as 1981–2010 should be represented and validated. Done means climr_downscale() can use the supplied climatology for downscaling without relying on the packaged PRISM data.

Written by the indexing model from the issue text.