Incrementing version number to 2.1
Correct the type declaration of 'coef_3rd_order' from 'integer'
to 'real (kind=RKIND)' in two places in mpas_atm_time_integration.F.
In order to force an update of the WRF physics tables to the corrected version of
the RRTMG_LW_DATA.DBL file, modify the checkout_data_files.sh script to look for
tables compatible with MPAS-Atmosphere version 2.1.