New boundary condition to support user-defined particle trajectory transformations at a boundary surface

[connoramoreno]

Description

It would be useful add support for users to define their own transformations for particle trajectories at surface boundaries. After a conversation with @pshriwise, this could take the form of a new boundary condition with a transformation matrix as an input.

As an example of this, I'd like the ability to accurately model stellarator half-periods in OpenMC. However, this would necessitate a different boundary condition at the planar ends of the half-period than is currently available; one that not only reflects the particle trajectory but also rotates about the surface binormal to account for the stellarator's helicity. This transformation, unique to stellarator geometries (I think), can be straightforwardly described by a transformation matrix, similar to what's done for the reflective boundary condition.

I'll work on an implementation and, if desired, submit a PR soon.

Alternatives

It would also be possible to add a stellarator-specific half-period boundary condition, but this general implementation is likely more broadly useful.

Compatibility

Would only add something new and not change the existing API in any way. That said, if implemented, it may be useful under the hood to redefine the reflective boundary condition as inheriting in some way from this transformation boundary condition.

[paulromano]

Thanks for opening this @connoramoreno. I agree that the more general we can make it, the better!