Change all extraction CodedConcepts to use CodeValue rather than CodeMeaning

[Ed McDonagh]

And we should be paying attention to the scheme designator too.

Fixing imports for the GE C-arm (issue #739) has highlighted how vulnerable using CodeMeaning is to spelling errors or lazy implementations. Plus the discussions for pydicom implementation of structured reporting (https://github.com/pydicom/pydicom/pull/824) that has highlighted that in theory, the CodeMeaning can be translated without the CodeValue changing.

We might want to save this change to when we are using pydicom 1.x, by which time they might have implemented the Structured Reports PR and we can take advantage of it!