Updates to the subgrid grounding line interpolation

- only direct assignment of array elements in interpolation scheme
  (e.g. in loop at point i,j only write into array(i,j), not array(i+1,j))
- x and y components of the fractional grounded to floating mask
  available for output (gl_mask_x, gl_mask_y)
- modification of basal roughness at grounding line also in cold mode