Talamini/feature/rate dependence

[btalamini]

Enables rate dependent material response in the solid mechanics module. These materials take an extra argument, the time increment, so the call looks like stress = material(state, dt, du_dX, params...);

I didn't want to break the interface (yet) and make users change all their existing materials, so I added a new method setRateDependentMaterial(...) to the solid mechanics module. We can revisit unifying them later once we have some more experience and feedback.

The results look good. Here is the force-displacement output of a new example app, uniaxial tension of a bar:

Other notable changes:

• Chris improved the build system files in the examples/ directory and made things clearer and easier to replicate for adding new examples
• Fixes a bug in the tensor symmetric 3x3 eigensolver