Update FV3ATM Cmakelists

[dustinswales]

Description

The CMakeLists.txt controls the atmospheric build configuration for several FV3 based UFS applications. This script will need to be abstracted to do either an FV3 or MPAS enabled builds.

Solution

The UWM CMakelists.txt supplies a pre-processor directive to control the dynamical core choice, FV3 or MPAS. The FV3 build shoud be unchanged other than to accommodate any codebase reorganization associated with the generalization of the atmospheric component.

[dustinswales]

@scrasmussen This has already been done in https://github.com/NOAA-EMC/fv3atm/pull/873/files, but we should pull out only the generalization/reorganizational relevant pieces (e.g No MPAS build just yet, I will add another Issue for that)