v5.1.0 (= NWM v2.0) Versioning

Purpose: Document how we will maintain the NWM v2.0 feature branch.

Notes:

1. Real semantic versioning numbers (and alpha and beta tags) are only used for WRF-Hydro version numbers. Those WRF-Hydro version numbers will be 5.1.0-alphaX and 5.1.0-betaX (details below) with NWM v2.0 production code being 5.1.0.
2. This NWM branch always merges into master.
3. Until the start of the production run, no development is allowed on master branch. All development not on the v5.1.0 branch must be placed on a feature branch (or branches) until this v5.0.1 branch reaches production (when we can reasonably expect no more major changes to the branch). Note that CI will be available for feature branches, so that feature branches must use local testing (talk to Joe or James about that if/when needed).
4. Do not change the files used on WCOSS without simultaneously updating all of a) the CONUS test files on cheyenne, b) the .version file in the repo, and c) the metadata in tests/config_file_meta/ (which reflects the namelists file structures while replacing the files with text including their md5sums and ncdump -h).
5. In the future, NWM releases and community releases should correspond. E.g., for WRF-Hydro v6.0.x the likely NWM version will be NWM v2.1 (or maybe v3.0, or maybe other).

Time\Branch	v5.1.0 ->	master
Hand-off to OWP	code_version= v5.1.0-alpha1 ... v5.1.0-alphaN nwm_version = v2.0	code_version= v5.1.0-alpha1+dev ... v5.1.0-alphaN+dev nwm_version = none
Start para run	code_version= v5.1.0-beta1 ... v5.1.0-betaN nwm_version = v2.0	code_version= v5.1.0-beta1+dev ... v5.1.0-betaN+dev nwm_version = none
Start prod run = tag v5.1.0	code_version= v5.1.0 nwm_version = v2.0	code_version= v5.1.0+dev nwm_version = none
After NWM v2.0 in prod		code_version= v5.1.1+dev ... v5.M.N+dev nwm_version = none