MultiPartOutputFile API fails when single part has no type

[peterhillman]

Either the API should inject 'SCANLINEIMAGE' type, or presence of type should be checked even in single part files

[cary-ilm]

@peterhillman is still still an issue worth considering?

[peterhillman]

I'll close this. Trying to create a multipart output file now checks that every part has a 'type' attribute even if only one part is written to the file, which is consistent behaviour for the using the MultiPartOutputFile class