HDF5: Correctly choose datatype of plotfile based on RD numBytes

[arnav-singhal]

Summary

If numBytes is 4, use H5T_NATIVE_FLOAT instead of H5T_NATIVE_DOUBLE. This fixes writing real values, as in ExaEpi.

Additional background

As far as I can tell, whichRD (and therefore whichRDBytes) does not change between levels, so I perform this check outside of the level loop, but if it is indeed possible for these values to change, I can move it inside.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[atmyers]

Thanks!