The parameters are now classes with attributes

Instead of dictionaries. This has the advantage that they can be nicely
documented with Sphinx. It should also be possible to thorow an error if a
non-existing attribute is written.
Implement a FrozenClass

This class does not allow new attributes. It will be used for the parameter classes.
Add a Dummy model problem
Remove shallow water options from configuration
Remove more options from the global config dict
Simplify ConstantFlowInitialCondition
Fix smeared test
Fix multi steady state test
Tidy up a bit