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.