Extend cnxvalidate to validate any NeXus file

[mkoennecke]

Currently cnxvalidate balks if there is no application definition. There is a desire to continue validating against the base classes then, with a hard coded empty application definition. Thus cnxvalidates scope is extended to a general NeXus file validation tool. There may be changes required to the recursion to support that.

[mkoennecke]

I added support for validating NeXus files without an application definition to cnxvalidate.
This has still to be reviewed before we merge it with master.

[mkoennecke]

Still to do:

• Turn current errors about missing root level attributes and application definition to
  warnings
• Handle the ignore* Attributes
• Check what happens with a non NeXus HDF5 file, it should come back with that it cannot find
  an NXentry

[mkoennecke]

It may be that MXML may be able to pull in default attributes for definitions from the mxdl.xsd

[mkoennecke]

With the ignored* attribute it may still make sense to report this as an informational message.

[mkoennecke]

There are two ways to implement this:

• Add code to SecondPassIterator() to recurse into additional base class groups find
• Make a FAT application definition which contains all that is possible from the base class definitions

There is probably little value in this: we can only find names which are not allowed.