Shell mechanics code needs to be updated

[vvedula22]

Shell mechanics solver in svFSI needs to be updated to model hyperelastic materials. Currently, we support only St.Venant-Kirchhoff material that performs poorly under compressive loads. The solver also needs to be tested for higher-order elements such as quadratic triangles, serendipity quadrilateral, etc. that were recently implemented but haven't been tested for shells. Potential applications are in valvular mechanics and FSI simulations, improved CMM, etc.

[vvedula22]

Made some progress on this. Fixed the current shell mechanics implementation for constant strain triangles.

Commit: vvedula22@ac5ee22

Tested it on, (a) bending of a plate subjected to normal load, and (b) mechanics of the aortic valve under transvalvular pressure gradient with contact between leaflet surfaces. Both the examples are now working and included in svFSI-Tests/09-shells.

TODO:
(a) Add hyperelastic modeling framework for shells under plane-stress assumption
(b) Extend to higher-order elements. The plate bending case worked for NURBS/IGA. Need to test on other higher-order finite elements.

[vvedula22]

Hyperelastic material framework is added in pull req. #96.

TODO:

1. Review and update documentation on shells in svFSI_master.inp
2. Add more examples for shells in [svFSI-Tests](https://github.com/SimVascular/svFSI-Tests)
3. Update contact model
4. Parallelize shells based on triangular elements