Physics needs to know if the "null" dycore is being used.

In general the physics in CAMDEN should be agnostic when it comes to the dycore being used. However, the "null" dycore likely will require special handling of certain physics input variables, as well as a potentially unique run sequence, at least during physics initialization. Thus the physics needs to know whether or not the null dycore is being used. There are two possible options:

1. Set an `is_null_dycore` logical at build-time, either through a CPP def or via auto-generated code. This will prevent the user from having to worry about setting anything themselves, and fits with the current CAMDEN configuration workflow, where the dycore is set at build-time.

2. Create a new namelist entry that notifies the physics whether or not the null dycore is being used. This namelist entry's default value will likely still be set during build time (or at least during any call to `buildnml`), but can be modified by the user at run-time via `user_nl_cam`. The advantage of this option, besides being run-time configurable, is that in the future if we need different versions of the null dycore it will be easy for the user to choose via the namelist. The dis-advantage of course is that the user could set the namelist value in-appropriately, but this may just require some extra value-checks in the source code to ensure nothing too catastrophic happens.

Please let me know if you have a preference for either option.