RF: use dcm_data get for dicomwrappers get

Confusingly, we were using 'getattr' on the underlying dcm_data, for
dicomwrappers 'get', and for `__getitem__`.  Use 'get' on dcm_data
instead.  Add tests.

This has the happy feature that you can now use 'get' and '__getitem__'
with tags, to get stuff from the dcm_data attribute.