DefaultDatasetService: require a DataTypeService

This fixes the following unit test failures:

* DatasetTest#testGetPlane()
* TestCursorSpeeds#testCursorSpeeds()
DefaultImageDisplay: make DisplayService optional

This fixes unit test failures in ImageCanvasTest.
DefaultDisplayService: simplify for loop
DataTypeService: add missing TODO
DefaultDataTypeService: add missing TODO
DefaultDataTypeService: fix section header