Easier visualization of curl for 2D vector fields

When working in 2Din the XY-plane, the curl of a field has only a Z-axis component. Visualization is easier if that Z-component is visualized as a scalar field instead of a set of parallel vectors. This MR adds a flag in the "Curl" plugin to make it possible to remove the X and Y components of the curl and create a scalar field instead of a vector field.

Example:

Let us take a 2D field which is the sum of an elastic plane P-wave (with zero curl) and a plane S-wave (with zero divergence)

We take the curl without this MR. All vectors point in the same direction.

We take the curl with the new "Z-only" option enabled (It is not, by default) and get a scalar field instead: