Boundless panel intersection

[dimitrivlachos]

Panel::get_ray_intersection checks whether the computed intersection
lies within the physical extent of the panel before returning a result.
This is correct for determining whether a reflection is genuinely
recorded, but is too restrictive when the intersection is needed
regardless of whether it falls within the detector boundary, like when
computing bounding boxes.

This PR adds a companion method that performs the same
ray-to-panel-frame projection but omits the bounds check, returning the
mm coordinate for any forward-facing ray.

• Add Panel::get_ray_intersection_unbounded, which projects s₁ into
  the panel frame via D·s₁ and returns the mm intersection for any ray
  with a positive depth component, without checking against
  image_size_mm_.
• Refactor get_ray_intersection to delegate to
  get_ray_intersection_unbounded and apply the bounds check on the
  result, eliminating duplicated projection arithmetic.
• Fix a typo in the return comment of get_ray_intersection (mmm →
  mm).

This allows callers that previously had to replicate the projection
arithmetic inline to use the Panel API directly, while preserving the
bounds-checked behaviour of the existing method for all current calls.

[jbeilstenedmands]

Thanks!