Merge branch 'feature-FO_execute_on_end' into 'develop'

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()

See merge request !24