Refactor data element classes

[ZviBaratz]

Currently, dicom_parser uses the dicom_parser.data_elements module to create individual classes for each DICOM value representation (VR) and implement custom parsing functionality. This decision was made in a rush with the intention to enable easier integration with django_dicom and more flexibility implementing custom parsing functionality. In practice, most VRs do not require any custom parsing functionality, and therefore the module mostly clutters and complicates the code base with numerous empty wrapper classes that do not add anything to pydicom's existing functionality. It would be much better if the existing functionality could be achieved without all the empty classes.