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.
RF: require dcm_data argument to wrappers

It was a bit confusing that we defaulted to an empty dictionary when
instantiating dicom wrapper objects without any input arguments.  Make
clear that the wrappers do need a dicom-info-providing thing.
Merge pull request #173 from matthew-brett/better-dicomwrapper-get

Better dicomwrapper get

DICOM sprint - simplify the interface to dicomwrappers by making wrapper 'get' defer to self.dcm_data.get, and similarly for __getitem__.

Also - make the dcm_data a required input argument to the wrapper to be clear that it is required for sensible functioning of the object.