Enable datatype specifications for each input variable

[spencerahill]

If a variable is a function of multiple variables, the user should be able to designate input datatypes for each one individually. This has come up for me when computing budget residuals: I need 3 hourly instantaneous data to compute tendencies, but some of the other terms in the budget were only output as monthly.

Need to think through more carefully, but I think the CalcInterface class could be a starting point for this. Essentially, each Var inputted to a Calc would need to be given the misc. dtype attributes. Would also need to figure out how the user specifies these.

Supposing #3 was ultimately implemented, the dtype specifications of a var would then be copied down to those needed to compute it, if that var itself isn't found on disk.