Need extensible image types

[mbeckerle]

The image types in NITF are a small set - no compression, jpeg, jpeg2000, etc.

Users may only have DFDL schemas for some of those. So the NITF schema shouldn't require them to be present. Instead, a set of dummy definitions for them should be placed in src/test/resources so that NITF can be used and tested in isolation, but in a "real" use, the classpath would hold the real schemas for the image types.

This is similar to the message_size element that was done in mil-std-2045 DFDL schema. https://github.com/DFDLSchemas/mil-std-2045

[stevedlawrence]

This was completed for jpeg since a schema exists for jpeg. All other image types are just treated as hex binary blobs.