Resolve "Task that creates input for Flint plot"

Closes #58 (closed)

I added the PlotData task. The output can be used as follows:

from ewoks import execute_graph
import matplotlib.pyplot as plt


result = execute_graph(...)
fig, axs = plt.subplots(2, 2, figsize=(10, 10), constrained_layout=True)
for data, ax in zip(result["plot_data"], axs.flatten()):
    ax.plot(data["x"], data["y"])
    ax.set_xlabel(data["xlabel"])
    ax.set_ylabel(data["ylabel"])
    if data["ylog"]:
        ax.set_yscale("log")
plt.show()

In reality Bliss will forward this to Flint.