Added static method to JP2Decoder for determining if an incoming input image...

Added static method to JP2Decoder for determining if an incoming input image is a valid JPEG 2000.  Changed ImageImporter to use this new method, and throw an exception if it cannot determine the type.  Added complete documentation to ImageImporter and all its children.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4142 41f8697f-d340-4b68-9986-7bafba869bb8