Upgrade wire connection code to handle grids cleanly

Changes forced by 'stretching test case' of building a model of a car passenger cell with a handheld transmitter inside. This requires meshed grids to connect cleanly, which requires 0,1,2,3 or 4 edges to be left 'open' and requires changes to the wire connection decision functions:

Revised sheet grid model to

thin_sheet(self, model, sigma, epsillon_r, force_odd = True, close_start = True, close_end = True, close_bottom = True, close_top = True, enforce_exact_pitch = 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
• the four 'close_' parameters determine whether or not the edges are 'sealed' with a final wire (if True) or not (if False) so that the grid can be joined to other grids without wires overlapping:
  • close_end = True completes the grid with a final end z wire at y = length/2
  • close_start = True starts the grid with a z wire at y = -length/2
  • close_top = True completes the grid with a y wire at z = height/2
  • close_bottom = True starts the grid with a y wire at z = -height/2
• enforce_exact_pitch: if True, length and height are adjusted to fit an integer number of grid cells of the specified pitch. If False, length and height remain as specified and the grid pitch in Y and Z is adjusted to fit the number of grid cells calculated from the grid pitch and force_odd value. Behaviour prior to V2.0.3 was enforce_exact_pitch.

"point_should_connect_to_wire" now also checks if wire to be connected is near a segment boundary, and preserves the wire (doesn't split it) if true