[PL/TH2M] Extend Jacobian with Laplacian's derivatives.

There are four Laplacian contributions to the pressure equations, LCpG and LCpC for gas pressure, and LWpG and LWpC for capillary pressure.

Some of the derivatives were already provided, but needed either a correction or extension, especially of the relative permeability and saturation parts.

See individual commits for details.

1. Feature description was added to the changelog
2. Tests covering your feature were added? Existing Liakopoulos and McWorther ctests.
3. Any new feature or behavior change was documented?