The dataset doesn't set the b2complement flag when the data is stored in signed 32bit integers

[Paolo Brandoli]

Pixels stored in signed integers should be force the flag b2complement to true. Currently when 32 bit signed integers are used the flag is not set.

[Paolo Brandoli]

- Fixes #74 (When the transfer syntax as implicit VRs then the VR for image data should be OW) - Fixes #73 (The Dicom codec returns black images when highbit=31) - Fixes #75 (The dataset doesn't set the b2complement flag when the data is stored in signed 32bit)

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