Motion along spherical surfaces

[mastrof]

Motion of microbes along spherical surfaces should be implemented along the lines of what I previously had in Bactos.jl (see mastrof/Bactos.jl#11 to implement the interactions and mastrof/Bactos.jl#50 for the neighborlist via CellListMap.jl).

This allows for different types of behavior along spherical walls, which is better to have than only the pathfinder approach. With this implementation, out-of-the-box microbes can:

• move smoothly along the surface until a tumble occurs (slide!)
• stick to the surface until a tumble occurs (stick!)

I will probably not implement the method to reorient as soon as they hit the surface (bounce!).
The interface to celllistmap will be provided to allow for easy addition of any custom methods by the user.

[mastrof]

Too specific to include as part of the library