ENH: functionObjects: call execute on last time step

• old convention was that on last time step it would only call end() and not execute()
• however this meant that e.g. the functionObjectProperties file did not get written
• and almost all functionObjects were doing an execute() inside of end()
• new convention: call execute() on last time step, just before doing end()

mentioned in commit f6d17d48

Status changed to merged