ensure index entries are comprehensive

[prjemian]

We found this morning that "offset_units" (in NXtransformations) is not in the documentation's index. Make sure all names in documentation appear in the Index. (field, group, attribute, and link names)

[prjemian]

see http://download.nexusformat.org/doc/html/classes/base_classes/NXtransformations.html#nxtransformations

[prjemian]

The item that triggered this issue, offset_units appears in the documentation like this:

.. index:: offset units (field attribute)

**@offset_units**: (optional) :ref:`NX_CHAR <NX_CHAR>`

  Units of the offset.

There is a substitution of for _ in the python code. Let's change this.

[prjemian]

Here it is:

https://github.com/nexusformat/definitions/blob/master/utils/nxdl2rst.py#L214

[prjemian]

and:

• https://github.com/nexusformat/definitions/blob/master/utils/nxdl2rst.py#L250
• https://github.com/nexusformat/definitions/blob/master/utils/nxdl2rst.py#L320