Improve dcm.py inheritance

[tomaroberts]

Related to #8 – need to improve class inheritance. Currently, the Dicom class is too MR biased.

TODO:

• [x] shift the add_module() calls to DicomMRI subclass
• [ ] need to ensure init_study_tags() and init_series_tags() are called after all the other tags in DicomMRI subclass are initialised
• [ ] replicate this structure with DicomCT subclass that calls modules relevant to NEMA CT Image Module Table