NetCDF HDF5 unit tests + bugfixes

These commits add unit tests for HDF5 encoded netcdf files (netcdf4). They expose these issues:
-  UnidataImageReaderSPI can not read HDF5 and NcML files, because the canDecode method is too strict (fixes included)
-  When reading from a file, the NetCDFReader throws an exception because a negative scale value is passed to a JAI scaledescriptor. (No fixes included.)

The patch includes some useful, small, binary netcdf files, as well as the CDL which can be used to generate them using ncgen.