Large sponge layer causes segmentation fault

[ooreilly]

If the sponge layer becomes too large, AWP segfaults.

To reproduce this bug, try --ND 50 (works) and then --ND 450 (doesn't work).

[ooreilly]

The sponge layer thickness must not exceed the number of grid points per GPU in each direction (NXT, NYT). The condition ND < min(NXT, NYT) must be met.