Add option to make marker-size user-defined

[jgieseler]

The symbols for the spacecraft have a fixed size at the moment. Provide an option for the user to define them individually, so that they can indicate e.g. how well an event is seen at each spacecraft. It probably is a good idea to provide a whole dictionary (per spacecraft!) like in the example at https://matplotlib.org/stable/gallery/lines_bars_and_markers/marker_reference.html so that in the same process things like color and shape can also be adjusted:

marker_style = dict(linestyle=':', color='0.8', markersize=10,
                    markerfacecolor="tab:blue", markeredgecolor="tab:blue")

Note that this will probably run into problems with markers=‘numbers’ or ‘letters’ because the position of the strings are hardcoded on the symbols!