Small changes to tiny expression for improved usability

[Lomholy]

Free-form text area

This PR adds a heavyside and gauss function to tinyexpr.c and force tinyexpr to evaluate exponentials from right to left.

This PR then implements a test of the two new functions in the relevant test instrument

Finally documentation of these small additions have been added documentation to Inhomogenous_incoherent_process.comp

This adds the features requested by me in #2461.

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Development OS / boundary conditions

MacOS Tahoe 26.4

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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!)

• 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

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[Lomholy]

Mcviewtest of the inhomogenous process component

Mcdoc of the inhomogenous_process component. For some reason the latex seems to not be loading correctly. Did I mess something up, or have you seen this before, @willend ?:

Mcdoc of test

[Lomholy]

Running the --c-linter gives warnings on the phonon_simple process:

But otherwise no warnings or errors from the linter.

[willend]

@Lomholy OK.

I think we should soonish have a discussion about “tinyexpr” - which I guess is by now already “something else based on tinyexpr” since there are local edits wrt. upstream https://codeplea.com/tinyexpr.

Since upstream is zlib license != GPL3 normal practice is in fact not to “just dump” a modified file in McStas.

A more correct approach would be to define/build a “derived package” from a fork of the original project ( could e.g. be the one I have already put in https://github.com/mccode-dev/tinyexpr ) and make a stand-alone package from that to be installed as a dependency.

( Having had another look at what is written upstream he sort of does hint at “just including in your own project” - but I think we could / should do something a little more streamlined that more easily allows updates from upstream developments… :-) )

[willend]

@Lomholy I can look at the \TeX later - or you can check that you are wrapping in the correct html tags as shown in https://github.com/mccode-dev/McCode/wiki/mcdoc-component-header under point 8?

[willend]

@Lomholy wrt. the TeX, something seemingly broke in mcdoc itself. Should not prevent merging here.

[willend]

TeX fixed, problem with webserver certificate

[willend]

Corrected TeX in mcdoc.

I will write a discussion item to exchange on a less hand-held solution for tinyexpr.

Merging.