Log a warning when a unsupported modality is read
Improve conversion of SliceImage to DICOM

It will now handle the creation of a DICOM object from a SliceImage that
was created from scratch (i.e. not loaded from a DICOM file).
Add a CR DICOM image sample
Remove support for CR images as projection images
Add support for CR images in RTKIT

These images are loaded in a 'CRSeries' in the 'Image' class. This class
is intended to be a 'general' image class, holding images of modality
'CR' (these film images can be either slice or projection type images,
without it being possible to know at load time).

Note that the CRSeries class is currently unspecced. Possibly the image
series classes will be reorganised at a later time.