Rename `gradient_criteria`

[vnmabus]

Currently the name gradient_criteria is used in the plot function to determine the values used to define a color gradient (e.g. the target values in regression, or the depth of the curves).

This name is not very good:

• Syntactically, it is plural, when it should be singular (there is a single criterium).
• This does not mention that it is a color gradient at all.

We should discuss alternatives and pick one, to change it. Ideally it should be coherent with the nomenclature in other Python packages. Current alternatives:

• color_gradient