Add new viscoelastic model for human myocardium

[ktbolt]

A viscoelastic model for human myocardium has recently been developed by David Nordsletten and Gerhard Holzapfel. We would like to incorporate this model into the solver.

I met with Will Zhang (U Mich) to discuss the code he shared with me from https://github.com/willwiz/fractional_model_test. I think I can use that C++ code as a guide.