Added ability to set the node class using a string

The string is then converted to a valid NeXus class.
Reordered contextual menu items
Enhanced the "Rename Data" action

Now, the node class can be changed, and a list of valid classes and/or
field names, as defined by the relevant NXDL file, is shown in a
dropdown menu
Updated documentation

To add info on the new NXDL capabilities.