Improve sub-plot titles

[golaz]

A couple of improvements to the sub-plot titles for model and observation:

• [x] Ability to specify a short name for the model instead of the full case name. The best place is probably the python parameter file.

• [x] Add climatology years for model data. This information is available from a global attribute in the climatology netcdf files generated by ncclimo, for example:

                :yrs_averaged = "61-70" ;

• [x] Add climatology years for obs data, also read from a global attribute in the netcdf file.

• [ ] Ideally, the obs name should also come from a global attribute in the netCDF file rather than being specified in the json file. This will minimize the chance of future mistakes.

[chengzhuzhang]

For model names, it's basically done, https://github.com/ACME-Climate/acme_diags/pull/109

1. added a new parameter 'short_test_name' for user defined model name, todo: update documentation to reflect this new parameter change.
2. added 'year_averaged' to print out as part of the subtitle.

For observational data names, it depends on the processes of generating data. Will be done in a follow up fix.