BUG: Have header keep original rescale slope/intercept values

[ebald19]

Relating to this issue

Issue

The header values for RescaleSlope and RescaleIntercept were being overwritten to convert the image density to HU. This caused a mismatch between original data and output data on write.

Proposed Changes:

• Remove overwrite of read in rescale data values in member variables [m_RescaleSlope, m_RescaleIntercept]
• move HU conversion to inside RescaleToHU function
• use RescaleToHU as member function which only rescales member data within function scope
• Adjust test cases to match correct RescaleSlope and RescaleIntercept values

These changes maintain the original header data, and use HU-corrected values only when converting the image data to HU. They also update the header tests to check that the values are maintained as original.

See below for the Scanco read original header values for the test images:

[dzenanz]

Looks good on a glance.

[mkuczyns]

@dzenanz: Erica (@ebald19) is working with our group over the summer to update the ITKIOScanco module. The PR looks good to me :)