Create method for setting visible value range in original (HU) values

[mlavik1]

The material has a visible value range, which uses normalised values (range: 0.0-1.0) relative to the dataset's min/max data vlaue.

It would be nice to add a method (to VolumeRenderedObject) that lets you set this value, by passing original data values (usually hounsfield unit).

The maths would be as simple as:
normalisedUnit = Mathf.InverseLerp(dataset.GetMinDataValue(), dataset.GetMaxDataValue(), hounsfieldUnit)

Do that for the min/max values, and update the material properties.