move bias and contrast from Image.set_colormap to Image.set_scaling?

[kswang1029]

wonder if we should move bias and contrast keyword arguments from Image.set_colormap() to Image.set_scaling()? Effectively when a non-0 bias and a non-1 contrast are applied, the resulting transfer function as seen as a grey curve is a combination of the selected scaling function (eg Square root), the bias function and the contrast function. What do you think?

[confluence]

Sounds sensible. We'll need to consider the logic around resetting the bias and contrast values (currently they're reset when the colourmap changes, which is not what the GUI does, and we should drop that). set_scaling should accept the special AUTO value (which is being added in #79) and handle it by calling the appropriate reset function. The scaling parameter to set_scaling should also be optional, so that the function can be used to change the scaling options without changing the scaling function, and the function-specific parameters should probably be set regardless of what the function is.

[I-Chenn]

Sounds sensible. We'll need to consider the logic around resetting the bias and contrast values (currently they're reset when the colourmap changes, which is not what the GUI does, and we should drop that). set_scaling should accept the special AUTO value (which is being added in #79) and handle it by calling the appropriate reset function.

To accept the AUTO value, we need that Auto constant validator created in #79. Since #79 is still being postponed, is it proper to create that Auto validator here and deal with the conflict later when #79 is merged?