Split plots builtins by base class

[bdlucas1]

This is a preliminary code rearrangement to make subsequent refactoring easier to do and easier to review. Plotting functions from plot.py that shared common base classes were pulled out into separate files by base class. Mostly a mechanical cut&paste operation, with a little manual cleanup to move variables defined at the top of the file.

[mmatera]

I think this is a good idea. My only observation is that the usual pattern for this seems to be to create a module mathics.builtin.drawing.plot, and then include inside plot.py->plot_base.py and the other files inside.

[bdlucas1]

Maybe that's where we'll end up, but at this point there is still some significant function in plot.py so creating a plot module would hide that significant function away in init.py which I think would be confusing (at least, it would confuse me). At this point I'd rather focus on what's going on in plot_plot.py, so maybe we can consider this a step along the way.

[mmatera]

Maybe that's where we'll end up, but at this point there is still some significant function in plot.py so creating a plot module would hide that significant function away in init.py which I think would be confusing (at least, it would confuse me). At this point I'd rather focus on what's going on in plot_plot.py, so maybe we can consider this a step along the way.

In that case, LGTM