Better include of template files

[gbrivady]

See the end of box.h. This is not ideal for two reason :

• Include path is explicit, which is not great if we were to move the template implementation to another place
• There is a recursive include in box.tpp so C++ LSPs don't complain (ymmv)

Same issue in rectangle.h/rectangle.tpp.

Ideally, someone should look up conventions or other projects that use template file so we can both:

1. Find a better way to split include and template files
2. Decide if the split is even a good or idea, or if we should just stick with a simgle .h or .tpp file.