don't copy RGF on deepcopy

[hexaeder]

This achieves similar behavior to normal Julia functions. Fixes #98.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API (Well, i would say it wasn't explicitly specified before)
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC. (I run the formatter, but the rest of the file does not coply yet. The new code does comply.)
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.