Informative error message when executing a Hessian calculation without loading or initializing the ensemble

[mesonepigreco]

When you ask for a calculation (free energy hessian, for example) when the ensemble is not loaded, the system crashes with a very non-informative error (about the forces that are a list and not a numpy array).

It should crash with a more informative error message, communicating to the user that he/she may have forgotten to load or calculate the ensemble.
This error has been spotted in #357