Add tests for divergence, vorticity, and q criterion

The gradient filter has the ability to compute divergence, vorticity, and q criterion. However, none of the gradient tests were actually testing the result of these computations. At best, one of the tests was checking that the arrays were created, but never actually looking at the values.

For one of the gradient tests, this change computes these 3 values and checks the results, which look reasonable compared to ParaView.