Add support for choice element

[domna]

In NXDL one can use a choice element to select out of two elements. We should also support this in this conversion tool.

Here is a link to the manual: https://manual.nexusformat.org/defs_intro.html#choice

[lukaspie]

I just tried it, it is actually already supported. You can write

choice:
  pass_energy(NX_FLOAT):
    unit: NX_ENERGY
  drift_energy(NX_FLOAT):
    unit: NX_ENERGY

which is translated through nyaml as

<field name="choice">
      <field name="pass_energy" type="NX_FLOAT" units="NX_ENERGY"/>
      <field name="drift_energy" type="NX_FLOAT" units="NX_ENERGY"/>
</field>

The backwards conversion does also work.

So this is just a documentation issue.

[domna]

Yes, but it is picking up a different syntax. It should be a tag choice, i.e.,

<choice ...>
...
</choie>

[lukaspie]

Ah right. Anyway, according to the manual, choice is only for groups, so it should not actually work for choosing between different fields of a base class, right? And it should also have a fixed name like pixel_shape in the example in the manual, so just choosing between two different names is also not supported.

[domna]

Ah right. Anyway, according to the manual, choice is only for groups, so it should not actually work for choosing between different fields of a base class, right? And it should also have a fixed name like pixel_shape in the example in the manual, so just choosing between two different names is also not supported.

Yes, I actually remembered this incorrectly. I updated my original comment on the drift_energy. However, I think it's also helpful to have the choice element for fields, so I opened an issue for it: nexusformat/definitions#1357

Anyways, I think we should still add the support in nyaml and pynxtools.