Standardize MMM plotting methods

[wd60622]

What should be required for plotting methods in the MMM? Here are some of my thoughts:

• method returns a fig, axes where axes is NDArray[plt.Axes] even with one element
• media transformations plots should allow subseting by channel
• Ability to pass axes to the plotting method
• Ability to change the colors of the associated curves

There should be a stand-alone plotting function for methods that will support minimal I/O. For instance, channel contributions should be passed in DataArray instead of a Dataset or InferenceData. The method can extract the necessary information from the DataArray and then call the plotting function. Reference: #821

This would help support other use-cases or general plotting needs that exist outside of the pre-defined MMM classes.

Related to #820