code: function addSource (row, col, mag) {
      var i, j
      for ( i = -2; i <= 2; i++ ) {
        for ( j = -2; j <= 2; j++ ) {
          if( row + i >= 0 && col + j >= 0 &&
              row + i < height && col + j < width) {
            u[(row + i) * width + (col + j)] += mag * gauss[ (i + 2) * 5 + j + 2]
          }
        }
      }
    }
docs: /*
  adds a new gaussian droplet to u
  at specified coordinates.