Enhance symmetry handling

[SebastianRuffert]

[lmh91]

Alright, let's deal with the IO and how it is stored in Simulation later.

Let's first get the following running:

using SolidStateDetectors
sim = Simulation(SSD_examples[:Coax]);

sym = MirrorSymmetry(30degree) # We need a constructor for this. 

Grid(sim)  # This should return a grid over the whole world. Thus, 360deg.
Grid(sim, symmetries = sym) # this should return a grid from 30deg to (30+180)deg with reflecting boundaries at both ends.