Add nexus definitions/files for beam path description

[RonHildebrandt]

Adding functionality for a description of a beam path
In total four NeXus definitions are added:

• NXopt_assembly
• NXopt_beam
• NXopt_element
• NXtransfer_matrix_table

Optical elements (NXopt_element) describe elements of a setup. A beam can be defined as object(NXopt_beam), which is associated with two optical elements and can be used to describe physical properties of a photon beam.
A unique path can describe the beam properties along a starting element (source) and an ending element (detector). The beam properties are modified by each optical element (e.g. attenuation, divergence). Matrices shall be used to describe this change of beam properties. These elements are part of the optical assemly (NXopt_assembly), in which the respective formalism is given.
For each formalism, there can be different matrices. These matricies are part of a large table, which contains all the necessary information to model the beam properties along a path. Each matrix table is associated for one optical element for one combination of input and output beams.

Unclear
The NXtransfer_matrix_table data structure is based on the NXellipsometry definition. Im quite unsure if this structure can be functional this way.

First PR
As this is the first PR by myself - Any advices for further PR and explanations for general rules are welcomed
Just did by to the best of my knowledge and belief.

[lukaspie]

Hi @RonHildebrandt, congrats on your first PR 😄 I wasn't sure if you wanted to have review yet, but this wasn't marked as draft. If you want to ask specific people to review, you can tag them in the upper right corner.

I have added some comments, mostly related to wording. However, there are also some design flaws:

• You need to define your base classes properly, i.e., as NXmy_base_class(NXobject).
• You need to define dimensions symbols.
• At the moment, you cannot extend another base class. This is what we have called base class inheritance, which was not in the original NeXus design. We will probably have this at some point, but for now, you must extend NXobject. If you want to extend NXbeam (and bring all of its fields and attributes), you have to manually copy all of them for now.

In general, all of these files should be in the contributed_definitions/nyaml folder because they are not accepted by NIAC for now. In addition, you should also make the NXDL files (by make nxdl) and make sure NXDL and NYAML are consistent.

[RonHildebrandt]

I have added adjustments of all files.
Still have to solve the failing checks. Is there a problem with the nyaml versions right now or should it be fine, if "make local" and "make nxdl" works?

[sanbrock]

LGTM