BUG: Fixed crash in vtkNRRDReader when attepting to read a 4D spatial file

Problem was that when scalar type was not supported, target memory area pointer for memcpy remained NULL.
Also reduced the number of cases when CanReadFile returns true but the reader cannot actually read the file.

The reader was really messy, so cleaned it up (smart pointers, variable definitions, formatting, etc), without any functional changes.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25105 3bd1e089-480b-0410-8dfb-8563597acbee