Generalize discrete hydrostatic balance computation in IGW

[valeriabarra]

Is your feature request related to a problem? Please describe.
In the ongoing investigation of instabilities when using 3rd-order-upwinding operator, it was recommended to debug/test this using the inertial gravity wave example with a stretched grid configuration.

To do so, we need to generalize the hydrostatic balance computation in the example to account for non-uniform grids.

Please link to any related issues/PRs.
This is a follow-up issue of issues #817 , #806 , and #803
Describe the solution you'd like
Generalize the discrete hydrostatic balance computation in the inertial gravity wave example and propagate the necessary changes in the other examples that use the same driver.