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")