Use object-oriented interface for PyPlot not global state

[fsaad]

It's a good habit to encourage users of Gen, especially as they write more complex code for plotting it becomes essential.

Using the object-oriented interface is recommended by the matplotlib documentation (the only reason the non-oo interface exists is for legacy compatibility with MATLAB's poor design). The OO interface is strictly superior in terms of code manageability.

Instead of

function render_trace(trace; show_data=true)
    xs = get_args(trace)[1]
    assmt = get_assmt(trace)
    if show_data
        ys = [assmt[(:y, i)] for i=1:length(xs)]
        scatter(xs, ys, c="black")
    end
    slope = assmt[:slope]
    intercept = assmt[:intercept]
    xmin = minimum(xs)
    xmax = maximum(xs)
    plot([xmin, xmax], slope *  [xmin, xmax] .+ intercept, color="black", alpha=0.5)
    ax = gca()
    ax[:set_xlim]((xmin, xmax))
    ax[:set_ylim]((xmin, xmax))
end;

figure(figsize=(3,3))
render_trace(trace);

We obtain the improved variant

function render_trace(trace; show_data=true)
    fig, ax = plt[:subplots](figsize=(3,3))
    xs = get_args(trace)[1]
    assmt = get_assmt(trace)
    if show_data
        ys = [assmt[(:y, i)] for i=1:length(xs)]
        ax[:scatter](xs, ys, c="black")
    end
    slope = assmt[:slope]
    intercept = assmt[:intercept]
    xmin = minimum(xs)
    xmax = maximum(xs)
    plot([xmin, xmax], slope *  [xmin, xmax] .+ intercept, color="black", alpha=0.5)
    ax[:set_xlim]((xmin, xmax))
    ax[:set_ylim]((xmin, xmax))
    return fig, ax
end;

fig, ax = render_trace(trace);

without any global fig or ax = gca() objects lying around.