Time blocking of various in --> out file numbers

[pgierz]

From @chrisdane:

Another feature request: that the user can give the number of years saved in one cmorized file, if e.g. annual, 10-year, 100-year, etc. files are wanted (per variable)

[pgierz]

This looks like it is handled (sort of) in pyormize.files.save_dataset, where the function _frequency_from_approx_interval takes a number (in days) and spits out something else. This function looks weird for a few reasons:

• Leap year and year are handled the same, so there is ambiguity
• The user should just be giving the string directly, I don't see why there is any need to convert anything.

So, in my view, the behaviour should look like this:

rules:
    - name: ...
      inputs: ...
      output_frequency: "10YE"

Where the string "10YE" is just any known resampling string