Resolve "Refactor code to get a clean source tree."

We should open the discussion on the refactoring issue. So have a look at the source tree under this branch, and please pay particular attention to the following:

• the python bindings will end-up in a separate directory --> python, i.e. we separate the core cpp code from wrappers
• we distinguish between demos (demo) and tests. Tests should end up as close as possible to the location that they're testing, see https://git.astron.nl/RD/EveryBeam/-/tree/2-refactor-code/cpp/test
• coefficient files end-up in the coeffs directory
• all kinds of utility scripts (most often related to coefficient conversions) end up in a scripts directory
• namespacing "more or less" follows the source tree. However, I haven't been very rigid in this as this would require, for instance that the custom typedefs live in a namespace everybeam::common. I think that's a bit cumbersome?

Any comments/feedback/criticism is very welcome @tol @offringa!

Closes #2 (closed), Closes #21 (closed)