Inline code in header files should probably be implemented in separate `*.inl` files

[nasso]

Some headers are getting way too hard to read because of all the inline code (e.g. World.hpp).

We could instead implement all the functions in a separate .inl file that is included at the bottom of the header file. This should make the code easier to read and document.

See also: https://stackoverflow.com/questions/1208028/significance-of-a-inl-file-in-c