Adding capability to use temporally subsetted stream files

The temporally subsetted stream files need to be handled by the CDEPS for especially high resolution input datasets such as ERA5. To test the capability in the current version of the CDEPS, I tested two different scenarios. To do that, i modified datm.streams.xml to include the only subsetted files,

/glade/p/cesmdata/cseg/ufs_inputdata/atm/datm7/ERA5/ERA5.TL639.2019.08.200618_subset.nc
/glade/p/cesmdata/cseg/ufs_inputdata/atm/datm7/ERA5/ERA5.TL639.2019.09.200618_subset.nc

and then i performed following tests;

**CASE 1:** I created a subsetted data based on the simulation RUN_STARTDATE. So, the data start exactly from RUN_STARTDATE and covers entire simulation period.

It seems CDEPS is working in this case and picking correct index (1) from the file for the first time step.

**CASE 2:** In this case, i generated a subsetted file that starts an earlier date than simulation RUN_STARTDATE i.e. 2019-08-28 and again covers entire simulation period. In this case, the first data read from input file need to be belong to index 24 (for 2019-08-29) because the data file starts 1 day earlier from simulation RUN_STARTDATE in this test.

In this case, CDEPS is not picking correct date from the file and still tries to get the field from index 1 rather than index 24. I think this is due to assuming having data for entire year (via start end end years in the XML definition) and fixed time interval. To have more generic implementation, CDEPS need to look at the dates (generated by using dataset time dimension - time unit, calendar etc.) not indexes.