Transfer function internals

## Use Case

Internally, histograms and LUTs are still represented in the 0-255 range, and the vole-app side jumps through some hoops to speak that language. Can we make this more technically robust?

## Solution

Allow histograms/luts to be defined in terms of absolute intensities, rather than a portion of the data range stretched over 0-255.
Three dimensions to consider:
- \# of bins in the histogram (currently 255)
- \# of elements in the lookup table
- Values of the control points

The "weak" solution to this ticket might look like just moving the translation down to the vole-core level from the vole-app level, so that the 0-255 range is entirely internal and opaque to clients. The "strong" solution could look more like actually allowing a variable number of histogram bins, LUT elements, etc. and handling that internally.