attempt to fix testSymmetry for GizmoMFV

test/testSymmetry.c wouldn't compile for the current version of GizmoMFV, it was looking for primitive structs in the part sturct.

Furthermore, the GizmoMFV specific particle comparisons routinely raised FPE errors when overflows, infinities, and NaNs were read in from the particle structs as floats. I re-wrote those checks to attempt a memcmp first, and if that failed, we then do a check for precision.