Different choices of types for data. Nexus format only requires NXnumber
for a larger number of its fields.

Test V20 file added. This is the 10th iteraton of the input file

Needed for instrument loading.
Also re-instated the findGroup() function in NexusGeometryParser as the refactored
version was disfunctional (segfaults)

In LoadEventNexus, after having checked if an instrument XML is found in the file, first check if the file contains geometry in
the binary nexus data before searching for the instrument name.
The file is checked using the LoadGeometry::isNexus() function and if it returns true, LoadInstrument is called with a
filename as argument rather than instrument name.

And also updated doc for the new bindary file format.