[Poro] Cleanup and refactoring of the App

[ipouplana]

📝 Description
This PR is meant to cleanup and refactor the PoromechanicsApplication as a first step towards the future addition of the two-phase flow in porous media formulation. That means that this PR must be merged before the two-phase flow formulation is added. All the previous tests pass in FullDebug mode.

🆕 Changelog

• The current formulation is moved into a folder named one-phase_flow
• The constitutive laws are split between two folders: continuum_laws and interface_element_laws
• All references to "water" are replaced by "liquid" to be consistent
• Some variables have been renamed to be more general
• Two variables: NODAL_WATER_PRESSURE_GRADIENT and NODAL_DAMAGE_VARIABLE have been removed as they were not used
• The time integration schemes input variables have been updated to be more general
• The new Poromechanics file paths have been updated in DamApplication too.