custom legends don't work in @plotter functions

[Thomas Gilgenast]

if a function decorated with @plotter includes a legend=True kwarg in its signature and uses this kwarg to trigger creation of a custom legend, then the @plotter decorator will fill in the 'legend' key of the plot_kwargs dict, and adjust_plot() will eventually call plt.legend() after the custom legend has been drawn, breaking the custom legend

an example of a function where this happens is lib5c.plotters.pca.plot_pca()

the proposed solution is to make the @plotter decorator treat this as a special case, and pass through legend=None to adjust_plot()

[Thomas Gilgenast]

fixes #43 using the solution proposed in the issue

→ <<cset c0ab7b7e7637>>