Add support for "Image Data File"

[proccaserra]

allowed in ISA specifications and Schema

• missing Class in model/datafiles.py
• implement and test
• provide example and document

Fall back on using Raw Data File for now.

[proccaserra]

Image File is among the allowed values for DataFile. and therefore supported.
see isatools/model/datafile.py and 'isatools/resources/schemas/isa_model_version_1_0_schemas/core/data_schema.json