Wrap image metadata in SWIG

ImageMetadata sgould be wrapped in SWIG so metadata dictionaries can be accessed from Python. This would allow metadata manipulations in Python pipelines.

The whole ImageMetadata API should be exported, along with the metadata key enums. In addition, some metadata classes should be wrapped so they can be manipulated from Python (e.g. RPCParam, TimePoint, Duration).

Maybe we could use SWIG typemaps to wrap some of the exported types directly to Python types.