[Users] GRHydro: NaN checks

[Christian D. Ott]

RE: GRHydro

All,

I would like to remove a bunch of NaN checks in Eigenproblem,
Marquina, Roe, Prim2Con, and Con2Prim that somebody put in for
debugging purposes (to track down a problem) and that are detrimental
to performance (I ran tests with and without them active). 

If a NaN appears, the code will still fail and standard debugging
techniques can be applied to track it down. No need to have these
permanently in the source code.

Any objections?

Thanks.

 - Christian