CUBA Keys for DEM problems

[roigcarlo]

Dear all, Since we can now use materials, the previously hardcoded variables in the Kratos DEM wrapper can now be configured by the user.

Most of them can be mapped directly with existing CUBA keywords, but we would need to add the following ones:

PARTICLE_COHESION
FABRIC_COEFFICIENT
CONTACT_TAU_ZERO
CONTACT_SIGMA_MIN
CONTACT_INTERNAL_FRICC

Also we would need to store a couple of string so we can know if a particle needs to be particle or part of a fiber (Maybe there is some metaclass for that matter right know?)
CONTINUUM_CONSTITUTIVE_LAW_NAME
DISCONTINUUM_CONSTITUTIVE_LAW_NAME

Also, we currently use CUBA.DENSITY as out internal PARTICLE_DENSITY, not sure if this is correct.

I ping @maceligueta and @pooyan-dadvand as well in case you need insight of which magnitudes this variables represent.