Add a reflection operator for grids (enhancement)

[jahargreaves]

It would be great to add an operator that can return a grid reflected in one or more geometric planes. I have this in my own BEM code, and it's easy to implement, but it's not a feature in bempp I don't think. It is very useful as - when applied correctly - it allows users to exploit symmetry and significantly reduce DOF count. I'm happy to get involved in helping code this feature.