Merge grid stacks

To merge grid stacks, you must merge the individual grids of the stack into a new stack.

For example, if you have two stacks, stk_a and stk_b, you would have the following structure:

   stk_a       stk_b
   stk_ac1     stk_bc1
   stk_ac2     stk_bc2
   stk_ac3     stk_bc3   (where stk_* are the component grids)  

  Grid: layer1 = MERGE (stk_ac1, stk_bc1)
  Grid: layer2 = MERGE (stk_ac2, stk_bc2)
  Grid: layer3 = MERGE (stk_ac3, stk_bc3)
  Grid:
  Grid: MAKESTACK stk_ab LIST layer1 layer2 layer3