DICOM reader and header nodes added

[borupdaniel]

Just a heads up for @rkrobison — I just added ReadDICOM and DICOMheader nodes to the development branch. Would like to release these as v2.2.0 in the near future as I think these could be really useful even without the DICOM writer node.

Couple of issues I wanted to ask you about before I tag the release:

• Any progress on reading Enhanced DICOM images correctly? If it's not a hard fix it would be great to work it in, but otherwise I'm fine with releasing the reader for Classic DICOM only to start.
• If you're able to run a super quick test that would be excellent. Preferable would be testing in a clean install of gpi/gpi_core to a new Conda env (along with PyDICOM), but anything you can do would be much appreciated! Seems to be working fine for me so I'm not too concerned about this one.