Support real*16 (and real*4) in GRHydro

[Erik Schnetter]

Support using precisions other than real*8 in GRHydro by removing all explicit references to double precision functions and constants, and using type-generic functions and constants instead.

In particular, use "one" and "half" as constants in some places, and use "abs", "max" etc. instead of "dabs", "dmax" etc.

Keyword:

[Frank Löffler]

Please apply.

[Erik Schnetter]

Applied.