Add new GE private tags

As found in:
http://www3.gehealthcare.com/~/media/documents/us-global/products/interoperability/dicom/computed-tomography/gehc-dicom-conformance_optima_doc1051015_rev3.pdf
Rework PixelFormat support for 64bits pixel

While it should theoretically possible to handle 64bits, the low level API of PixelFormat currently prevents it (see GetMin/GetMax signature).
So properly discard the case.
Remove an assert which could be triggered

In case lossy compression method was lost attribute was missing. See bug #343