Method to compare two z-files generalized to compare n z-files

[kkappler]

We have been using a one-off method in transfer_function/plot/comparison_plots.py called compare_two_z_files

def compare_two_z_files(
    z_path1,
    z_path2,
    label1="",
    label2="",
    scale_factor1=1.0,
    scale_factor2=1.0,
    out_file="",
    show_plot=True,
    **kwargs,
):

but we want the ability to compare aurora to emtf to matlab so comparing three makes sense.

In general we want to be able to compare an arbitrary number of these files (with reason, say 6) in order to do sweeps over parameters in the config file.

Ideally, this would take z-file, or the mt_metadata TransferFunction class as input.

*We may want to make a separate method for casting z_file to mt_metadata TransferFunction and then the plotter only needs to support a single interface