Support time-series analysis over a subset of the output data

[xylar]

Currently, the the code will only work as expected if config option timeseries_yr1 = 1 and timeseries_yr2 is set to something beyond the end of the current run (e.g. 9999), which are the default settings. If timeseries_yr1 > 1, the code crashes, likely because mpas_xarray is expecting the time series to start from year 1 (it isn't currently making use of timeseries_yr1). If timeseries_yr1 = 1 and timeseries_yr2 is set to be less than the total duration of the run, the timeseries analysis may be performed beyond the desired end time. This is because we add one file with date stamp beyond the final date to be sure we're not missing any important output.

The time axis of the xarray data sets should be sliced to be consistent with these two new config options in each timeseries analysis function.

[pwolfram]

@milenaveneziani and @xylar, we will have some other potential mpas_xarray changes too. Is this something we need to fix now or can we wait until it is time to make the other mpas_xarray changes?

[pwolfram]

To be clear, this would be for the item:

• mpas_xarray needs to support different approaches to specification of time output for AMs (Calculation of datetime to be unified in the future  #33, Read paths from streams and config from namelist #38, others?)

[xylar]

@pwolfram, it's not clear to me whether the problem here is that mpas_xarray needs to be updated or just that the calls to mpas_xarray need to be updated. I suspect the latter, in which case this fix is independent of other changes to mpas_xarray.

[xylar]

So I think most (perhaps all?) of what is needed to address this issue is to change lines like the following:
https://github.com/MPAS-Dev/MPAS-Analysis/blob/develop/mpas_analysis/ocean/ohc_timeseries.py#L90

    time_start = datetime.datetime(yr_offset+1,1,1)
    time_end = datetime.datetime(yr_offset+1,12,31)
    ds_yr1 = ds.sel(Time=slice(time_start,time_end))

to look more like the corresponding lines in the climatology computations, e.g.:
https://github.com/MPAS-Dev/MPAS-Analysis/blob/develop/mpas_analysis/ocean/ocean_modelvsobs.py#L129

    time_start = datetime.datetime(yr_offset+climo_yr1, 1, 1)
    time_end = datetime.datetime(yr_offset+climo_yr2, 12, 31)
    ds_tslice = ds.sel(Time=slice(time_start, time_end))

[milenaveneziani]

This was addressed by #48, right?

[pwolfram]

Please reopen if this is a premature closing @xylar.