Enhance Regularized Coulomb sliding Law

[mperego]

This PR:

• Fixes mismatch of units in Regularized Coulomb sliding law (Now the flow factor has consistently units of Pa^{-n} s^{-1}).
• Adds capability to provide a Constant Flow Rate to be used for the sliding law.
• Renames Ice Softness to Flow Rate for consistency with naming in Viscosity evaluator.
• Imports the bed roughness field from MPAS
• Save the effective pressure computed in the BasalFriction evaluator into the mesh (if field is provided)
• Adds two tests using Regularized Coulomb law for Humboldt glacier