FIX or DOC: Consistent user-facing stress convention

[bobmyhill]

The initialization of stress fields (i.e., in models involving elasticity) follows the engineering convention (-ve = compression), while the output (i.e., the stress postprocessors) is in geological convention (-ve = tension).

We should either make the user-facing conventions consistent with each other or at least document the conventions used prominently in the manual.

Opened for discussion.