Add another 8-bit field in vtkDICOMValue for overflow.

This is specifically for delimited sequences stored in OB or UN VRs,
which can potentially surpass 4GB in size.  By catching the overflow
bits when NumberOfValues exceeds 32 bits, sizes up to 1024GB can be
stored.

For other VRs, the overflow byte acts as a sentinel that will be set
if a value was allocated with a length of more than 4GB.

I've only done some of the necessary Parser changes, and none of the
necessary Compiler changes.  Also, lots of testing is necessary.