Improving the index. Distinguish different categories of attributes. Index NXDL elements.
Merge branch 'master' into corrections
Merge remote-tracking branch 'upstream/master' into corrections
"data field" -> "field" in nxdl2rst.py
one more "data field" -> "field" in nxdl2rst.py
"data field" -> "field" in several manual sections.
Merge pull request #324 from jwuttke/corrections

Improve index entries. Distinguish between different categories of attributes.