Adding some examples to NXpdb

[yayahjb]

In an effort to improve the documentation of the relationship between imgCIF and NeXus I am working on some examples in NXpdb.ncdl.xml in base_classes. What I have so far is in branch hjb_code_camp_2021. I will keep working on this and report the status during the code camp closeout next Monday.

[prjemian]

Not seeing a NXpdb class at this time.

@yayahjb Does this refer to an existing class (such as NXmx) or some new work?