Replace constants in shr_const_mod?

There is a new initiative from [ESPC](https://earthsystemprediction.gov/) to create a [physical constants dictionary](https://github.com/ESCOMP/PhysicalConstantsDictionary) with published, internally consistent sets of physical constants which can used by modeling systems.
I think we can adopt this dictionary by creating a module of constants which can then be 'published' by shr_const_mod (using them in shr_const_mod and declaring them public).
This would enable selecting a physical constant set at build time but none of the component models would have to make changes. Ideally, all component models would use these constants.