Resolve "Rewrite exponential in terms of abstract classes"

Description

Change the exponential_apply_batch routine to take an abstract hamiltonian class and instances of the batch class as arguments. For this to work, two new methods were added to the batch class, clone_to and clone_to_array, that take care of allocating the memory to the correct type.

News snippet

Change the exponential_apply_batch routine to take an abstract hamiltonian class and instances of the batch class as arguments.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #217 (closed) Depends on !734 (merged) !716 (merged)