Adds blitting plot

[StephenNneji]

Adds a class to facilitate a blit plot in RasCAL, this class can be tested as shown below. The object creation should do a normal plot then subsequent updates will use blitting if the background has not changed. This leads to a decent speed up between the first draw and subsequent draws.

 class TestPlot:
    def __init__(self, block=False):
        self.block = block
        self.closed = False
        self.blit_plot = None

    def __enter__(self):
        self.figure = plt.subplots(1, 2)[0]
        self.figure.canvas.mpl_connect("close_event", self._setCloseState)
        self.figure.show()
        RATapi.events.register(RATapi.events.EventTypes.Plot, self.plotEvent)

        return self.figure

    def _setCloseState(self, _):
        self.closed = True

    def plotEvent(self, event):
        if not self.closed and self.figure.number in plt.get_fignums():
            if self.blit_plot is None:
                self.blit_plot = PLotSLDWithBlitting(event, self.figure)
            else:
                self.blit_plot.update(event)

    def __exit__(self, _exc_type, _exc_val, _traceback):
        RATapi.events.clear(RATapi.events.EventTypes.Plot, self.plotEvent)
        if not self.closed and self.figure.number in plt.get_fignums():
            plt.show(block=self.block)

Usage will be

with TestPlot(block=True):
    problem, results = DSPC_standard_layers()

[DrPaulSharp]

All looks good, and some good speedup. Just a few (hopefully) straightforward things to look at.