If we don't then we get a crash in the FileLoaderRegistry that tries
to cast to an incorrect type.

Refs #8177

This avoids having to declare a new interface when a new loader type
is added.

* Rename HDFDescriptor to NexusDescriptor
* Rename {HDF,NonHDF} -> {Nexus,Generic} in FileLoaderRegistry
* Rename DECLARE_HDF_FILELOADER_ALGORITHM ->
  DECLARE_NEXUS_FILELOADER_ALGORITHM

Refs #7461

This better encapsulates the file checking so that other places don't
have to worry about the desciptor stuff.
Refs #7263

It appears that we can't start the FrameworkManager during static
initialization or the destruction of the FrameworkManager instance gets
instance gets confused.

Refs #7523

The FileLoaderRegistry can now be asked to search its list of loaders
for the best that matches a given file. It passes along a *Descriptor
object to minimize the number of times the file is opened.

Refs #7263

and also defines two types of loader, NonHDF and HDF.
Refs #7263

Currently based on string names.