Major bugs removed.

Two quite important bugs have been removed for this release.
1) The electric potential is set to zero unless defined otherwide. Before it was set to 0.3 V for some odd reason, which screwed things up a lot.

2) The electric field bitmaps are now a lot more accurate. The programme now checks enforces the fact that the E-field across a conductor is zero. Before this may have not happened within a pixel of the edge of a conductor.