Change how metadata is assigned to `plot` objects

Problem to solve

Proposal

Users will be able to assign attributes both directly on the Plot objects or the instances of its subclasses:

s = Serializer(fig1)
s.plot.id = plotids.figure_ids[0]
s.plot.axes[0].xunit = "second"
s.plot.axes[0].yunit = "meter"

and also through methods on the Serializer class like:

s = Serializer(fig1)
s.add_plot_metadata(id=plotids.figure_ids[0])
s.add_axis_metadata(s.plot.axes[0], xunit="second", yunit="meter")

The methods Serializer.add_plot_metadata and Serializer.add_axis_metadata contain required arguments (e.g. axis) and optional arguments (e.g. id set by default to None).

It is important to determine which arguments:

• are necessary
• should be required vs. optional