[GeoMechanicsApplication] Clean up / simplify the calculation of the permeability update factor

[avdg81]

📝 Description Simplified and cleaned up the implementation of the members that compute the permeability update factor.

🆕 Changelog

• Made the member functions const.
• Use the return type to return the functions' result.
• Removed redundant usages of else keywords.
• Use std::pow rather than C-style pow.
• No need to instantiate a class to call its static members.
• Only pass what is needed to compute the permeability update factor.
• Reduced the scope of a local variable.
• Renamed local variables to comply with the Kratos Style Guide.