Add vr:XX field expander

[jstorrs]

Description

This is adds a vr: field expander that allows fields to be selected by VR. This allows targeting based on content. For example vr:PN will select all Person Name elements. Or vr:TM will select all Time elements without selecting Date Time elements.

Note: endswith:Time will select some fields that are DT (such as AcquisitionDateTime)

Checklist

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] My code follows the style guidelines of this project

Open questions

[vsoch]

heyo! Did you have any more thoughts on this design? I think it's good to go in for a first shot - we had a request in #219 for the feature so it would be good to have!

[jstorrs]

I agree the latest "select:vr" version is good to go.