Tests: Replace problem functions postTimeStep/preTimeStep
Functions with such names are due to the version 2 way of implementing simulation flow control. In version 3 such functions should not exist because
- the function name doesn't say anything about what the function does
- the functionality in those functions probably doesn't belong in the problem
The function might be removed by moving the functionality into the main file, or renamed to some appropriate name if the functionality really belongs to the problem (connected to BC/IC/Sources or maybe an analytical solution).