union system updated for McXtrace 3

[ebknudsen]

Including new Compton and Powder scattering process

Free-form text area

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

This PR adds work done for HALO-Xray in terms of implementing the Union system also for McXtrace 3 and include Compton and Powder scattering processes.

It is currently in a draft stage (i.e. not cleared for merge), using the old Union structure with Geometry_functions.c etc. in separate files. This is intended to be merged into a union-lib.c style description.

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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:
Debian buster

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[ebknudsen]

also this description does not yet contain surface refraction

[willend]

@ebknudsen great, thanks! As discussed elsewhere I will see if I can merge / cut / paste / add some of the improvements from the neutron side later. (Will very likely not happen this side of new years...)

[ebknudsen]

@willend Thanks - this will come at its own pace. Remaining to be done:

1. real regression test instrument(s)
2. adding in the malloc protections in union-lib
3. switching over to union-lib description.
   ... other things that I can't think of right now

In the longer term it could be desirable to split union-lib into a common and a neutron/X-ray specific description, to avoid falling out of sync, but that's not for now/this PR

[willend]

Agree on all of the above

[willend]

@ebknudsen mcxtrace-conda-basictest (both with openmpi and mpich) for some reason have decided to hang during very early initialisation… No clue what it is ATM.

[willend]

@ebknudsen found it / worked around. It seems the default conda-solver is simply overwhelmed/hung. Have for now installed mamba and used that…

I guess it is time for McCode to make use of something like https://github.com/mamba-org/setup-micromamba in the CI instead