Identify pairs of cells that are adjacent.
A two column matrix with each row containing a pair of adjacent cells.
Extracts the indices of those cells that are connected (e.g. cells i,j that have a non-zero value in the transition matrix).
Cell numbers are unique indices of cells in the original grid. By convention, cell numbers start with 1 in the upper-left corner of the grid and increase from left to right and from top to bottom.