Move relperm driver

[dkachuma]

Building on the constitutive testing infrastructure established in PR3883, this pull request moves the RelpermDriver use the same framework.

• Updates the driver to inherit from ConstitutiveDriver rather than directly from TaskBase.
• Automatically handles saturation sweeps for 2-phase and 3-phase models, ensuring the sum of saturations remains valid (<= 1.0) for 3-phase evaluations.
• Introduces a historicalSaturations XML parameter to correctly test drainage and imbibition departure curves by populating historical extremum phase volume fractions.
• Consolidates related input files into inputFiles/constitutiveDriver/, adds new ATS integration tests for various model types (Table, Brooks-Corey, van Genuchten), and provides full Sphinx documentation alongside a working XML example.
• Adds sphinx documentation for the relative permeability driver.