Pu time increment call back into energy functions

[btalamini]

Small addendum to Craig's commit.

The dt argument was meant to stay as an optional argument to the mechanics functions. The intent was to pass a value in every time you call them, giving you the opportunity to use different values (say for adaptive time stepping). This isn't documented anywhere, so there wasn't a way for Craig to know that. There is an example in the app examples/uniaxial_dynamic/UniaxialDynamic.py.

The time interface is a little clunky, you can see how the project started with quasi-statics. We could consider adding the current time as an argument to the density functions (like dt is), which would make it easier to write time-dependent loads.