New capability to build model with polaris setup or polaris suite

[cbegeman]

New behavior for polaris setup ... --model ${MODEL} -p ${PATH_TO_EXE}:

• If executable does not exist in ${PATH_TO_EXE}, build the model there from e3sm_submodules/components/${MODEL}
• If e3sm_submodules/components/${MODEL} does not exist, git submodule update
• A new flag or flags should be created for the purpose of the nightly suite (and other uses):
  • do a clean build of the model even if the executable exists
  • update the submodule even if it is already checked out

[xylar]

I have an as-yet-untested version of this for Omega (but not yet MPAS-Ocean) on a branch: https://github.com/xylar/polaris/tree/automate-model-builds