Instructions for merging two 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_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: MAKESTACK stk_ab LIST layer1 layer2 layer3