WIP: Real world value mapping

Rebased changes from : https://github.com/nipy/nibabel/pull/143

Please see the comments there.

Uses Real World Value Mapping SOP class - see : ftp://medical.nema.org/medical/dicom/final/sup103_ft.pdf

These changes need tests.  Also need thought about scaling of different volumes, and relationship of scaling to usual DICOM scaling with RescaleSlope and RescaleIntercept