Work on ESS source description / comparison with MCPL inputs

[willend]

Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:

The instruments included in this PR are related and forms the basis for comparing output from ESS_butterfly and the legacy target division MCPL inputs available from https://public.esss.dk/users/willend/MCPL/

1. ESS_butterfly_Guide_curved_test: Run for a given sector, beamline (%Example uses S2
2. Get a source file from https://public.esss.dk/users/willend/MCPL/ (e.g. S2.mcpl.gz)
3. Optionally use ESS_MCPL_generate_filtered to generate S2_filtered.mcpl.gz
4. Run comparisons via ESS_butterfly_MCPL_test

(e.g. use mccoplot for co-visualisation of 1-dimensional monitor outputs)

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Declaration of use of AI-tools

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

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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:

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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 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 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.

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[willend]

A first series of plots comparing ESS_butterfly_Guide_curved_test and ESS_butterfly_MCPL_test across the “available” beamports is now available at https://essbutterfly.mcstas.org/2026/