make GRHydro_Macros usable by other thorns

In #1213 knarf wrote:

We need a mechanism to make GRHydro_Macros.h usable by other thorns and especially other languages. If used as is currently I cannot write an extra thorn setting things to atmosphere conforming to these new parameters - especially not in C, unless I essentially re-implement the logic - which would be bad code-duplication. This was no issue so far because the logic was so simple. With this patch this would no longer be true.

Keyword: GRHydro