Adds solve, post function skeletons for processes and calling them from main.

This adds skeletons for solve() and post() functions for a Process class and in the GroundwaterFlow.
A process iterator for ProjectData is added for calling the initialize(), solve(), and post() functions.
The call loop is placed in main.
I think there is a more general iterator access implementation possible to hide the concrete container class. I am unsure if we need a general implementation later on. For the moment I would leave the implementation as it is. 
@norihiro-w Is this for you acceptable?