New C++-oriented `rdmconst.h` header

[shawn]

I recreated the intent of `rdm.h` into a new, more C++-oriented, `rdmconst.h` file. Notes about the file:

1. Uses `constexpr` instead of `#define`s.
2. Fixes some spelling mistakes from the standard by introducing new constants, but keeping the old constants and tagging them with `[[deprecated("spelling")]]`.
3. Puts everything into an `rdm` namespace.

Feel free to make comments and suggestions. Consider this a beta version.

The file is attached.

[shawn]

I've updated to a new version with more constants from E1.37-7. This one is `rdmconst-beta2.h`.

[bootress]

I'm replying to this way late since Scott reminded me about the forums last night, but I love this!!

One thing to note - using the [[deprecated]] attribute makes this usable in a minimum C++14 environment. We do encounter older toolchains than that, especially in the RDM world, so it might be worth adding that fact as a comment.