DICOM images with fixed RescaleSlope != 1.0 cannot be imported without conversion to float

[Sethur]

OS: Windows 10, x64
ImageJ: 153k

Currently, DICOM images with a fixed RescaleSlope != 1.0 for all slices cannot be imported without conversion to float, because the import logic handles every slice separately and has no way of knowing wether all slices will have the same RescaleSlope in the DICOM header.

This unfortunately leads to a slower import process for DICOM volumes with many slices even though most DICOM volumes (especially for CT and MRI) use fixed values and could be imported via calibrated 16-bit unsigned integer.

Pull request #122 fixes this issue by adding a new DICOM preference that allows the user to state that his DICOM RescaleSlope and -Intercept is fixed in imported volumes, allowing the import logic to avoid converting every image to float.