`...classes.friction.coulomb` and `...classes.friction.coulomb2` are the same

[lawrenceabird]

The initial ISSM Python implementation of frictioncoulomb.py and frictioncoulomb2.py appears to be the same. This has carried through to the pyISSM implementation (pyissm.model.classes.friction.coulomb and pyissm.model.classes.friction.coulomb2).

See frictioncoulomb.m and frictioncoulomb2.m for correct implementation and update accordingly.