Implemented ReflectiveShell as optional boundary

[ehlertdo]

Hi,
I needed a reflective boundary in my 3D simulations but in the form of a sphere instead of the currently available ReflectiveBox. Maybe this feature could be of more general interest since astrophysical environments are often more spherical than they are boxy.
Note that the shell reflects in both directions, i.e. outside particles are kept out and inside particles are kept in.
Best,
Domenik

PS: I copied the distance() and normal() functions from the Sphere class in Geometry.cpp but probably there's a more elegant way to do this with inheritance.

[JulienDoerner]

Hey @ehlertdo,

Thanks for sharing your implementation. In general, the tool looks good for me.

I have tested it and found some deviation when using large steps. I guess this is caused by the position where you calculate the normal. This is given at the current position which is not necessarily a position on the sphere. Would you have an idea how to fix this, or can this be neglected in regular cases?

Beside this I would ask you to add your changes to the Changes.md and maybe you can provide a simple test case as a notebook and as unit test.