222 n rendering layers in overlay

[MattClarkson]

A summary of things to look out for.

• Internally, 5 rendering layers instead of 3. I didn't see the benefit of putting them in an array, because they are a mixture of image renderers and VTK renderers, each with different purposes, so you would not be iterating through a list.
• Background, Foreground and Overlay made sense in a 3 layer model. I kept the API as close as I could so, default settings would revert to the old behaviour. As such, it should be a drop in replacement for applications that already use it like BARD.
• I did run the code through "black" formatter, default settings, to get PEP 8 style.
• As Python doesn't do private/public/protected, you may find client code or unit tests referring to internal member variables. This is bad practice anyway, so I tried to provide accessor functions.
• But mostly, I tried to keep the API changes minimal.

[thompson318]

Looks good to me and is passing CI, thank you.