This PR adds a Rectangle property to Heatmap to make it easier to define the edges of a heatmap using a rectangle in coordinate space.

Note that Extent does something similar, but defines the rectangle containing the centers of each cell, and is one cell smaller in size than Rectangle

• resolves Heatmap: Manually defined Extent does not produce the expected output #4552
• resolves Heatmap: How to display a heatmap over a given X and Y range #4550

Extent = new(-5, 5, -5, 5)	Rectangle = new(-5, 5, -5, 5)