Imaging Event object property issues

- `:ie_modality` is mapped to `ac:captureDevice`, which makes no sense. Should be changed. 
- Need to supply official labels and descriptions for DICOM terms.
- `:lens_make` and `:lens_model` should map to actual exif terms, not morphosource terms with exif prefixed on them.
- `:physical_object_id` is mapped to `morphosource:objectID`. This should be `morphosource:physicalObjectID`.