Add DICOM attribute RTROIInterpretedType as optional parameter to add_roi and ROIData

[rheg49]

RTROIInterpretedType stores information about the type of the contoured volume. Possible values can be found here: https://dicom.innolitics.com/ciods/rt-structure-set/rt-roi-observations/30060080/300600a4
One should be able to pass this parameter to add_roi so that the desired type of ROI can be passed to ds_helper.create_rtroi_observation(roi_data) within add_roi.

(follow-up issue to #70: after that, the RTStructMerger maybe could be changed to the usage of add_roi instead of just appending ROIContourSequence, StructureSetROISequence, RTROIObservationsSequence from one RTStructure to the other one)