rewrite coo_plot and plot.PCA into g()

[vbonhomme]

• that will make use of the new layer dispatching prepare_dots + do_with_args
• should make all plotters much easier to maintain and powerful to use
• gathers #121 and #130 that will be closed

a working version can be found in Momocs:::g(). Other plotters will be progressively deprecated.

[vbonhomme]

delayed and possibly a lot of work for not so much gain.