Introduce NXemission_line base class

[mretegan]

The PR introduces a new base class to describe an emission line. This base class will be used in the updated XAS definition (see #1352)

Because this may be of broader interest, and to make the review of the forthcoming XAS application definition easier, this base class is being submitted as a separate PR.

A question that came up in our previous discussion (XraySpectroscopy#8) was how to group together multiple emission lines. NXcollection was discarded as it is not validated. Another proposed alternative was NXobject (XraySpectroscopy#8 (comment)) or using a new container class NXemission_lines, which would result in an HDF5 structure looking like this:

/
├── entry:NXentry
│   └── emission_lines:NXemission_lines
│       ├── line_1:NXemission_line
│       │   ├── name = "K-L3"
│       │   └── energy = 5414.72   (units="eV")
│       ├── line_2:NXemission_line
│       │   ├── name = "K-L2"
│       │   └── energy = 5405.5    (units="eV")
│       └── line_3:NXemission_line
│           ├── name = "K-M3"
│           └── energy = 5946.7    (units="eV")

[mretegan]

The PR now adds three more base classes.

• NXemission_lines which is a container for grouping multiple NXemission_line https://nexus-definitions.readthedocs.io/en/add-nxemission-line-class/classes/base_classes/NXemission_lines.html
• NXauger_line describes non radiative Auger decay https://nexus-definitions.readthedocs.io/en/add-nxemission-line-class/classes/base_classes/NXauger_line.html
• NXelement defines a chemical element https://nexus-definitions.readthedocs.io/en/add-nxemission-line-class/classes/base_classes/NXelement.html, which is required for a proper description of the emission and Auger lines.

/
├── entry:NXentry
│   └── emission_lines:NXemission_lines
│       ├── line_1:NXemission_line
│       │   ├── element:NXelement
│       │   │   └── symbol = "Cr"
│       │   ├── name = "K-L3"
│       │   └── energy = 5414.72   (units="eV")
│       ├── line_2:NXemission_line
│       │   ├── element:NXelement
│       │   │   └── symbol = "Cr"
│       │   ├── name = "K-L2"
│       │   └── energy = 5405.5    (units="eV")
│       └── line_3:NXemission_line
│           ├── element:NXelement
│           │   └── symbol = "Cr"
│           ├── name = "K-M3"
│           └── energy = 5946.7    (units="eV")

[phyy-nx]

From Telco Apr 29: general approval was received. There were rumblings of linking to external databases instead of having copies of things such as the periodic table, but the consensus was that this could be ok.

There are implications to other base clases, e.g. NXchemical_composition which has an ELEMENT field of type NXatom, and in contributed, NXsubstance. This should be discussed more, such as should we introduce a new base class, NXelement, or should the XAS application definitions use NXatom instead?

[mretegan]

After looking at NXatom, I believe it should be possible to use it instead of creating a NXelement.

[phyy-nx]

Great, @mretegan will you provide a new version then?