feat: add axis argument to plot_alpha

[evanr70]

It is sometimes desirable to add alpha stackplots to existing figures. If multiple alphas are passed, multiple axes must also be used. A check is added to make sure there are an equal number of alphas and axes.

Changes:

axes=None -> no change (fig and axes created)
axes=single_axis -> wrap in a list
axes=multiple_axes -> check same number of axes as alphas