updates to Fitter.custom_plot

[jvavrek]

1. option to plot without drawing the additional fit info panel
2. remove hardcoded margin settings
3. remove savefname kwarg
4. associated notebook and docstring updates

[jvavrek]

From the updated fitting.ipynb, the new custom_plot (using a user-defined figsize, disabling the fit info panel, and calling plt.tight_layout()) now looks like this:

[jvavrek]

Should we also get rid of the title* kwargs? They only appear in the following line

# Set fig title
if title is not None:
    fig.suptitle(
        str(title), fontweight=title_fontweight, fontsize=title_fontsize
    )

which is normally left to the user to handle.

[markbandstra]

Should we also get rid of the title* kwargs? They only appear in the following line

# Set fig title
if title is not None:
    fig.suptitle(
        str(title), fontweight=title_fontweight, fontsize=title_fontsize
    )

which is normally left to the user to handle.

Yes, let's leave it to the user, so long as they can access fig.