This PR adds a simple Multiplot class for creating figures containing multiple Plot objects. Refinements to the API will be made in subsequent PRs.

This is only useful for saving image files. This type of plot is not supported in the user controls. To achieve this type of functionality in an interactive app, place multiple controls on the window.

Plot plot1 = new();
var sig1 = plot1.Add.Signal(Generate.Sin());
sig1.Color = Colors.Blue;
sig1.LineWidth = 5;

Plot plot2 = new();
var sig2 = plot2.Add.Signal(Generate.Cos());
sig2.Color = Colors.Red;
sig2.LineWidth = 5;

MultiPlot mp = new();
mp.AddSubplot(plot1, 0, 2, 0, 1);
mp.AddSubplot(plot2, 1, 2, 0, 1);
mp.SavePng("test.png");