Parse and unpack attributes into attribute-specific Golang data structures.

[suyashkumar]

We should parse and unpack Elements into native data structures that represent the elements. gradienthealth/dicom-protos already lays the foundataion to generate golang-native messages that represent each attribute, so this repository should populate those data structures. In other words, the below code should happen here and should likely be autogenerated

	// Code to unpack PhotometricInterpretation from a DataSet
	el, errFind = ds.FindElementByTag(dicomtag.PhotometricInterpretation)
	pi, errGet := el.GetString()
        // errors should be handled
	photomericInterpretation = &attrPB.PhotometricInterpretation{Value: pi}