Create geos-geomechanics package with geomechanics classes and functions from geos-posp and update dependencies

[paloma-martinez]

As a first step of code refactoring proposed in EPIC #54, this PR will consist in moving the basic classes and functions relative to geomechanics processing to a dedicated package.

Open question
The packages are currently independent, but the refactoring will bring some dependencies inside the repository.

• Should we keep the possibility to build each package manually or should the build be unified ?

The dependency might be especially problematic when specific python version will be required (evoked in #58 for the case of paraview plugins package geos-pv)