[improve] colormesh

[Mc-Zen]

This PR

• Adds an option to include an external image into a colormesh which is useful for very large meshes. Through min, max, and map, the colormesh instance can behave just as if it drew the mesh itself − including when colorbars are generated for the mesh.
• Adds a parameter colormesh.align which determines how the mesh rectangles should be aligned at the x and y coordinates.
• Adds the possibilty to specify edges of the mesh instead of points by passing x and y coordinate arrays which are one larger than the respective dimensions of the two-dimensional z array.

Examples for all three features are included in the documentation.

Closes #79 by allowing to include a colormesh that was generated by an external tool.