Helpful for debugging clang problems

Fixes the compilation problems, I hope

This is weird..

Fixes a lot of compiler warnings

Had cast to wrong type

Python part still needs to be fixed, almost there, compiles in clang again

Add some forceeval and type double where possible

Thought it would maybe help with compilation

But not clear why this changes anything...

The key point is to speed up compilation, and make it easier to develop, by using the packaged C++ interface more extensively

Add multipolar contributions

Not the problem, it was the pow(x,1/3), where x < 0