Include basic bpch diagnostics in AC_tools for NetCDF too

[tsherwen]

Include a set of core diagnostics for NetCDF GEOS-Chem output, like are present for bpch output.

• Get tropospheric burden (akin to AC.get_trop_burden for bpch files)
• Get surface concentration (" " AC.get_avg_surface_conc_of_X/AC.get_2D_arr_weighted_by_X '' ")
• Get Ox budget summary (" " AC.get_POxLOx " ")
• Get OH average ( " " AC.get_OH_mean " ")
• Get CH4 lifetime ( " " AC.get_CH4_lifetime " " )
• Get area weighted average conc. ( " " AC.get_CH4_mean/AC.molec_weighted_avg/AC.get_avg_trop_conc_of_X " " )
• Get species deposition (" " AC.spec_dep/AC.get_wet_dep " ")

This will require updates to return xr.datasets in the same form as in NetCDFs. Currently, the datasets/arrays created follow Met Office (non ESMCF/COARDS format)

• Get LWI lifetime ( " " AC.get_LWI_map " " )
• get model datetime (" " AC.get_gc_datetime " " )
• Some unit conversions ( " " AC.species_v_v_to_Gg " " )
• remove stratosphere (" " AC.mask4troposphere " ")

These functions are present in the separate GEOSChem_nc.py file (linked below). And the existing files for working with bpch output have been moved to GEOSChem_bpch.py

https://github.com/tsherwen/AC_tools/blob/master/AC_tools/GEOSChem_nc.py

Inc. test in unit testing for these and some example files in the remote

[tsherwen]

As shown by calls in function 'get_general_stats4run_dict_as_df', most of this is now implemented.

Still to do:

• Get OH average ( " " AC.get_OH_mean " ")
• Get CH4 lifetime ( " " AC.get_CH4_lifetime " " )
• Get Ox budget summary (" " AC.get_POxLOx " ")

[tsherwen]

This issue was closed as part of #116.

The updates were completed as part of the git merge from the Dev branch (#116) for work related to NOx budgets for ARNA campaign work.