[GeoMechanicsApplication] Split ApplyK0ProcedureProcess into a header and source file

[rfaasse]

📝 Description
Minor PR to split the ApplyK0Procedure class .hpp file in a .h and .cpp file.

[avdg81]

Thanks for splitting the original .hpp file in a header file and an implementation file. This makes the code easier to read, since the class's interface has been nicely separated from the implementation details. Well done Richard!