V2.0.2 Revised grid model, add example, revise plotting

Revised sheet grid model to

thin_sheet(self, model, sigma, epsillon_r, force_odd = True, close_end = True, **dimensions)
where

force_odd forces an odd number of wires, ensuring one central z wire and one central y wire cross at x=0, to make attaching other objects easier
close_end creates a final end z wire - use close_end=False to allow copying and joining meshes together without wires overlapping
model now has height symmetric around zero (as was the case for width)
Created example of dipole with capacity end points for loading, using the grid model

Updated plotting functions a little (gain range)