Top-level verification methods.

[tyralla]

Currently, the HydPy class, which provides top-level access to many other functionalities, does not offer any general verification methods for checking certain aspects of complete HydPy projects at once. Instead, one needs to visit the relevant child-objects and check their state manually or, if available, call their unique verification methods (for example, method verify of class Parameter). We should improve, standardise, and complement the available low-level methods and make them easily accessible via corresponding HydPy methods.

[sivogel]

A check for sequence files would also be nice. Currently there is no check anywhere that the number of entries is equal to the number of time steps defined in the ASCII header.