Add in option for CLMGSWP3 for datm
Add in option for CLMGSWP3 for datm. Also add in capability for driver namelist generation to merge CAM and CLM drv_flds_in namelists together,
and abort on a conflict. Add in testing of driver namelist. Work on SSP test and add SPL testcase. Add option to Namelist.pm->merge_nl to die on
conflict. change notes for Namelist.pm to be "!#" instead of "#!". Fix namelist test to work. Remove messing with drv_flds_in file in
preview_namelists.
Test namelist changes: showed that namelists are identical for ALL tests (all machines, all compilers, all test lists) in cesm1_5_beta06
Test status: bit-for-bit
Fixes: #324, #315, #56
User interface changes?: None
Code review: Design review a few months ago with: Ben A., Mariana V., Jim Edwards, Bill S., Erik K.