Initial commit of Phonon_BvK_PG and test instrument

[lefmann]

Free-form text area

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

Initial commit, but this is not yet cleaned up

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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 a new component file

  • I have ensured that naming of parameters are in the style of existing components. (Please check the McStas or McXtrace NOMENCLATURE docs.)
  • I have ensured that component parameters are in the usually units of McStas or McXtrace (SI + neutron/x-ray 'usual' units)
  • 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 included a corresponding example instrument and will fill in the new instrument section below
  • I have used the mccode-clangformat tool to apply the standard McCode component indentation scheme
  • My new component is added within the contrib component category

• 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 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

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[willend]

@lefmann I have shortened the “checklist” above to contain only the relevant tick-marks.

I will now see if I can reformat the c-snippets from J. Avery to become a more standard set of McStas %include files.

[willend]

@lefmann yikes!

I now have running prototype on Windows - hacked together just now… Loads of edits to both the Avery snippets and your comp code… Nasty process but now produces data:

[willend]

Supplementing the above rewrite of eigen.h/c to use mccode-complex-lib (which is tested to work correctly / identically to the original) I also put an intermediate version of the component here:
Phonon_BvK_PG.comp.TXT

Status is that this comp runs across platforms - but finer details of the main omega_q functions seems to have broken in the process… Output is different… (AI was used for some automation in replacing +-*/ operators by mccode-complex-lib` wrapper functions…)

[willend]

Identical output on unix and windows with the last commit