Limit a field to a spatial region.

Input parameters:

1. Fl → Field to clip.
2. Bn → Spatial boundary. Can be either a closed surface or an infinite plane.
3. In → If true, the 'inside/outside' behaviour of the boundary is inverted.
   •  True → Invert
   • False → Regular

Output parameters:

1. Fl → Field clipped to new boundary.