Improving the index. Distinguish different categories of attributes. Index NXDL elements.
refs #320
Update NXxbase.nxdl.xml

refs #317
fixes #316
Merge branch 'development' of https://github.com/nexusformat/definitions into development
Merge branch 'master' into corrections
XML typo
restore entry for NXvalidate (Java GUI)

Somehow, NXvalidate was removed from the documentation but still referenced in the h5py example.  Since NXvalidate is still in the code repository (<code>/applications/NXvalidate), it should be represented in the manual.
more explanation

additional content from the ``nxvalidate.1`` man page file from NXvalidate
re-insert

inadvertently omitted from #303
Merge remote-tracking branch 'upstream/master' into corrections
remove "link" attributes

fixes #334
"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.
Merge branch 'master' into development
Merge branch 'master' into development
emphasize early on that NeXus is used for both raw and processed data
reduce displayed figure sizes here, remove unnecessary "The"