Can't reverse non-iharm3d B field

[bprather]

Originally from Angelo: the option 'reverse_field', which flips the orientation of the B field when loading a file, is only implemented for iharm3d files.

This should be documented, or replicated to all (and all future) loaders, or moved outside the file loading machinery to some step afterward (keeping in mind that the field 4-vectors depend on the newly oriented 3-vectors).