Implementation of Hoek-Brown model in MFront

This merge request is created for the implementation of the Hoek-Brown model in MFront. A Jupyter notebook has been prepared in which the derivation of the model is described. Furthermore, I added the mtest files regarding the uniaxial, biaxial, shear, and hydrostatic tests. This implementation has been done based on the C2 continuous approach, which is used to modify the generalized Hoek-Brown function to avoid the singularities at the corners of the octahedral plane.