Add constrained mixture material model

@schwarz-e has a C++ implementation of the constrained mixture model (CMM) that takes the deformation gradient and returns the stress tensor and tangential stiffness matrix. Currently, this is called from a Python code.

We have a unique opportunity to take this CMM implementation and put it into svFSIplus as a material model, which hasn't been done before in any large-scale FEM code. The challenge is that we need to store a bunch of information on each integration point, which is currently all implemented in a vessel struct. Also, we need to test if the linearization of the stress tensor is correct.

I think this could be an interesting first svFSIplus project for me.

PS: Happy issue #2!