updates of solver performance destroy pimple controls

With commit 0ff86ee2 the management of firstIteration/finalInteration migrated into meshState. However, there is folded into the solverPerformance dictionary which is cleared with setSolverPerformance if the time index changes.

Options:

• ad hoc preservation of firstIteration/finalInteration when clearing the solverPerformaanceDict
• move firstIteration/finalInteration into a "controls" sub-dictionary and/or place in the main mesh state dictionary.

Reported by @Mattijs