revisit problem_post hooks and document

[zingale]

Currently there are 4 ifdefs for the problem post hooks:

• DO_PROBLEM_POST_TIMESTEP
• DO_PROBLEM_POST_RESTART
• DO_PROBLEM_POST_INIT
• DO_PROBLEM_POST_SIMULATION

Only the last of these is controlled by a make option (USE_POST_SIM).

Usually the problem-specific implementation of these is in Prob.cpp.

We should revisit whether we really want these ifdef-ed out or if we just have simple stubs always defined. For the latter, we might need to update some problem setups.

[zingale]

We also need to document this functionality