Describe the behavior you would like added to mBuild

fix_orientation : bool or list of bools
Specify that compounds should not be rotated when filling the box

Right now, it looks like no rotations are possible if fix_orientation=True
It would be nice to be able to limit rotations about a certain axis, but allow them around another.

Describe the solution you'd like
Perhaps fix_orientation can be a list or tuple
(x_axis, y_axis, z_axis)

And the argument could be passed as:
fix_orientation = (True, True, False)

I'm not familiar enough with packmol yet to understand how this would be implemented within packmol's constraints. Maybe someone with more experience will have some thoughts or advice?

Additional context
This came up because I'm using mb.fill_box where I want to keep everything planar, so fix_orientation = True, but I would like to allow rotations about the z-axis.