Fix zeroing of memory when getting coordinates in electrogram calculation.

In calculating the gradient of 1/R, the coordinates of all quadrature points are utilised. In less than three dimensions, GetCoords does not re-populate the higher dimensional coordinate (which should be zero). Since means these coordinates were not reset, but were used during the calculation, they accumulated values from the previous egm points resulting. This MR fixes this memory issue by resetting coordinates to zero for each egm point.