Prevent concatenation of "static" frequency variables?

[angus-g]

When files and their variables are added to the database, we make an attempt to guess the output frequency of the variable. In some cases, we mark a variable as "static", indicating that it shouldn't be interpreted as a timeseries. It might make queries a bit nicer if we enforced this:

• select only a single file containing the variable, so we don't need to force nfiles=1
• and/or don't try to concatenate static variables
• don't try to concatenate data of different frequencies -- I don't know if this would break any analysis, where people don't mind if monthly data has a few daily sections?

[aidanheerdegen]

FYI A recent update to the explorer (https://github.com/COSIMA/cosima-cookbook/pull/211) made static a special case and enforced n=1 as the solution.

A more general solution would be nice, but static still needs to be a special case in the explorer as there is no valid date range.