Create a new userfile model for HDF files

[prioux]

Typical extension: see wikipedia at https://en.wikipedia.org/wiki/Hierarchical_Data_Format

Viewers: just report some information from the content of the file using the h5stat and h5ls commands (if installed). only show a message if the file is compressed (these tools don't handle gzipped HDF files).

Once this is done, maybe make the MINC2 model in cbrain-plugins-neuro inherit from HdfFile.

Name things properly, e.g.

irb> 'hdf_file'.classify
 => "HdfFile" 
irb> 'hdf_file'.classify.underscore
 => "hdf_file"