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.

As a side-effect, this also allows delimited SQ values to exceed
2^32-1 items.

All other values are still limited to a length of 2^32-2 as per the
standard.