Get CROP to function separately from natural vegetation mode

Currently the modes for crop and natural vegetation are tied together (SP, BGC, CN, BGC-CROP, FATES etc.). You can ONLY turn CROP on if use_cn is being turned on. So you can't use it with SP or FATES -- at ALL. CROP is already on different land-units, but the switches are setup for the overall vegetation mode and not to separate out crop versus natural vegetation.

I think in order to do this, we'll need to allow the overall switches to be more flexible, but we'll also need something like a patch level filter that says: on_this_patch_you_run_CN, on_this_patch_you_run_FATES, on_this_patch_you_run_SP. I think we'll still want the ability to run the "normal" modes of SP (without crop), BGC-Crop, and FATES (without crop), so these would be new capability that could mix and match how natural vegetation is handled separately from crop.

At the CESM CTSM-FATES discussion we decided that having FATES model natural vegetation and BGC-Crop model crops, is a critical feature of our CTSM-FATES v1 model version.