Add function to write+read restart file

[MartineDeVos]

eWatercycleII: add a function that writes a restart file at the current model time, and read the file to actually do the restart. Starting from a restart should still be handled to feed the appropriate configuration file to the initializer function.

[hrajagers]

Use cases:

• check pointing
• iterative solver

[ipelupessy]

The alternative to this is being able to restart using the bmi getters and setters (which offers more flexibility). The rationale of seperate function for writing and reading the state is that its quite hard in practice to achieve complete coverage for the BMI getters and setters (and its difficult to achieve bitwise consistent restarts this way) ..