Mesh bugs

[Lomholy]

Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:
Fixes the bugs mentioned in #2421 .

The errors in the mentioned bug were two fold. Firstly, when placing two boxes such that neithers vertices were within the other, the algorithm for checking what geometries overlaps with which failed.

The fix for this was to add an extra check in the overlap algorithm, to check if the edges of a mesh overlaps with another geometry.

Secondly, when placing a mesh in relation to one arm that is rotated, and placing the master in relation to another arm, the algorithm for checking overlaps would also fail. This was due to the rotation of shell points in the allocate_shell_points function being performed after moving the shell points to the current position, rather than before. This has been fixed.

A new test instrument that previously failed on both these accounts have been added. Thank you to Sam Lambrick for finding these bugs!

Furthermore this PR performs some Quality of life improvements to the Mesh code. Most notable among these is the switch from representing vertices as 9 double arrays, to 1 Coords array, and the addition of an error messages in case the mesh file cannot be opened.

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:
MacOS Tahoe 26.4.1

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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 a new 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 ensured that basic use of the instrument is OK (e.g. it compiles?)
• ... and provided reasonable default parameters in that instrument that produce reasonable output
• ... and maybe even added a %Example: line to describe expected behaviour
• I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised
• My new instrument is added within the examples hierarchy in a folder in the style of examples/ESS/New_stuff/New_stuff.instr
• My new instrument has a new, unique filename, not clashing with existing example instruments
• My new instrument requires a data/input file. If the datafile is specific for my instrument I have left it in the same example folder, but if general use I have placed it in the global data folder.

• My PR is meant to fix a specific, existing issue

  • I have indicated the issue number here:
  • I have added documentation for the fix and possible side effects

• My contribution contains something else

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

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[Lomholy]

Here a mcviewtest screenshot with the new Test instrument:

And mcdoc for new Test instrument:

[Lomholy]

Union mesh mcdoc:

[Lomholy]

@willend @mads-bertelsen This is almost ready for getting review. I only have one issue which seems to not be connected with this patch. Using the --c-linter on the new test instrument, I get these warnings:

[willend]

@Lomholy, thanks for the update. There seems to be a merge-conflict in union-lib.c:

[Lomholy]

@Lomholy, thanks for the update. There seems to be a merge-conflict in union-lib.c:

You are very welcome.
The merge-conflict should be solved now.