Improve input variable handling

At the moment, all metadata is for input variables is replicated every time an index uses this input variable. Given that we only use a couple of different input variables, such as tas, pr, tasmin, and tasmax, this duplication of metadata like cell methods and standard names is unnecessary.

To better deal with input variables and prepare for future extensions, we should add a new, separate registry of input variables and only refer to that where we need the input variables in the indices.