Revert to not allocating fluid cons vars if fluid is not active?

[brryan]

There are some difficulties getting radiation to work right in all cases if the fluid is off when not allocating fluid cons variables, so I changed the code in #104 to always allocate both fluid prim and cons vars. Maybe in the future we want to think a little more carefully and stop allocating fluid cons vars if we don't have fluid fluxes.

[Yurlungur]

I think it's fine to allocate fluid cons, if we need them, so long as the fluid is frozen and not evolved.