Richer description in check_consistency function

[dario-coscia]

Is your feature request related to a problem? Please describe.
PINA modules heavily rely on the check_consistency function. This function is used to throw errors when passing an invalid instance or class object. The error message, however, is very short and, most of the time, unclear:

>>> from pina.solver import PINN
>>> PINN(2, 3)

ValueError: int must be <class 'pina.problem.abstract_problem.AbstractProblem'>.

Describe the solution you'd like
It would be nice to have a richer and more informative error message, something like:

1. subclass=true but instance passed

ValueError: You passed <wrong_instance> instance, but a <true_class> class was expected. Please pass a <true_object> class or a derived one. 

2. subclass=true but wrong classe

ValueError: You passed <wrong_class> class, but a <true_class> class was expected. Please pass a <true_object> class or a derived one. 

3. subclass=False but class passed

ValueError: You passed <wrong_class> class, but a <true_instance> instance was expected. Please pass a <true_instance> instance. 

4. subclass=False but wrong instance

ValueError: You passed <wrong_instance> instance, but a <true_instance> instance was expected. Please pass a <true_instance> instance. 

Describe alternatives you've considered
Maybe we should wrap this into a more structured class for handling errors in PINA.

[GiovanniCanali]

Hi @dario-coscia, I share with you the need to update error handling raised by check_consistency. We could think of implementing our own class for errors, in general.

[GiovanniCanali]

Fixed in #601 and merged in #615.