Add NXxasproc Concept, Represented and Instance variables

[patrick-austin]

Closes #20

Happy to merge without review, but if anyone has any feedback please let me know.

• Added nxxasproc to the curie_map
• Added Conceptual, Represented, Instance variables for all required fields: https://manual.nexusformat.org/classes/applications/NXxasproc.html
• Add cdif:Reference for the InstanceVariable-source of all NXxasproc Instance variables

Like NXxas, there is a single path in the hierarchy for the "signal", which in this case is the absorption co-efficient https://manual.nexusformat.org/classes/applications/NXxasproc.html#nxxasproc-entry-data-data-field. Unlike NXxas there is no other variable to denote whether this is transmission, fluorescence, reference etc. There is also not a distinction between Mu and normalised Mu as there was for XDI. To allow for this, have added Mu-Any as a Concept.

[scman1]

We should prefer domain vocabularies over general ones. In our case, the preferred vocabulary for terms should be the IUPAC goldbook, for chemical element, chemical reaction, etc. The paper Guidance on how to use the IUPAC Gold Book as a canonical source for textual definitions in chemical ontologies describes how the goldbook can be used for this purpose.

[scman1]

Another relevant dictionary which we might use is the Online Dictionary of Crystallography (ODC). For instance ODC contains definitions of XAS, EXAFS and XAFS. Altough, I am not sure how far should we go to define every term used.