[FEATURE REQUEST] Add OH metrics output for benchmarks

[msulprizio]

Reproduce the GAMAP metrics output in GCPy so that we do not need to generate with IDL. Ideally, we would compute and compare global mean OH, MCF lifetime, and CH4 lifetime.

[msulprizio]

The calculations for MCF and CH4 lifetimes require the time-in-troposphere diagnostic output from GEOS-Chem. This is currently only saved out to bpch diagnostics and needs to be implemented in History. @yantosca is working on that now.

[yantosca]

The time in troposphere netCDF diagnostic is now implemented in commit geoschem/geos-chem@c01871c.

A 1-day zonal mean of ND54 (bpch) vs. FracOfTimeInTrop (netCDF) looks like this:

[msulprizio]

This is now pushed in 509cb4f.

We still need to apply fraction of time in troposphere to calculation to include only purely tropospheric boxes. This will be added once the 12.6.0 1-month benchmark with the new FracOfTimeInTrop diagnostic is available.

[msulprizio]

This calculation now also works for GCHP but the values are off. For example,

### Global mass-weighted OH concentration [1e5 molec/cm3]                   ###
### Ref = GC_12.6.0; Dev = GCHP_12.6.0                                      ###
###############################################################################
  Ref          Dev          Dev - Ref    % diff     
  12.213840    60.125135    47.911295   392.2705

###############################################################################
### MCF lifetime w/r/t tropospheric OH [years]                              ###
### Ref = GC_12.6.0; Dev = GCHP_12.6.0                                      ###
###############################################################################
  Ref          Dev          Dev - Ref    % diff     
   5.339524     0.000000     0.000000     0.0000

###############################################################################
### CH4 lifetime w/r/t tropospheric OH [years]                              ###
### Ref = GC_12.6.0; Dev = GCHP_12.6.0                                      ###
###############################################################################
  Ref          Dev          Dev - Ref    % diff     
   8.980075     0.000000     0.000000     0.0000

Further investigation is required.

[yantosca]

We should probably implement this as a HISTORY diagnostic in GEOS-Chem, so that we can obtain the output for GCHP as well. This is documented in GEOS-Chem pull request https://github.com/geoschem/geos-chem/242.

As a stopgap measure, I also have implemented a GCPy function (mean_oh_from_logs.py) to strip the 1-year mean OH from the log files and to print them. But this will not work for GCHP.

I will close out this issue, as it seems that further work will need to be done as a GEOS-Chem diagnostic as described in the issue above.