[NOT READY FOR MERGE] Union refactoring of scattering while loop

[Lomholy]

Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:
This PR aims to improve the readability of the Union_master component. The PR is limited in scope to only the scattering while loop inside Union_master.comp.

To perform this refactoring I propose two things:

1. We separate all non-primary logic paths, out into separate functions (Currently placed in Union_master for my own simplicity, but will be moved to union-lib.c).

2. When determining a non-primary logic paths, we use wrapper functions, that clearly indicate the intent of the branching in logic path.
   An example of this is using:
   if (volume_is_only_absorber(Volumes[current_volume])) {
   Instead of
   if (Volumes[current_volume]->p_physics->number_of_processes == 0) { // If there are no processes, the volume could be vacuum or an absorber
if (Volumes[current_volume]->p_physics->is_vacuum == 0) {

3. (Maybe) We attempt to adhere to a standardized naming scheme for the functions implemented, such that all inhomogenous paths are called "inhomogenous_insert_what_function_does();".

---

Declaration of use of AI-tools

• Please add a checkmark here if you used AI-tools during the work for this contribution
• Further, please describe how / where and for what the tools were used:

---

Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:
MacOS Tahoe 26.5.2

---

PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My contribution includes patches to an existing component file

  • I have used the mcdoc utility and rendered a reasonable documentation page for the component (please attach as screenshot in comments!)
  • I have ensured that basic use of the component is OK (e.g. an instrument using it compiles?)
  • I have used the mctest utility to test one or more instruments making use of the component (please attach mcviewtest report as screenshot in comments)
  • I have used the mccode-clangformat tool to apply the standard McCode component indentation scheme
  • I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised

• My contribution includes patches to an existing instrument file

  • I have used the mcdoc utility and rendered a reasonable documentation page for the instrument (please attach as screenshot in comments!)
  • I have used the mctest utility to test the instrument (please attach mcviewtest report as screenshot in comments)
  • I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised

• My work touches / adds to the runtime lib code (.c,.h etc in multiple locations

  • I am have added reasoning and documentation for the change below
  • I am attaching test output in the comments

• My contribution contains something else

  • Explanation is added in free form text above or below the checklist

---