Remove unpack statements to save operations

Description

Batches should enter and leave hamiltonian_apply_batch and exponential_apply_batch in the same state. If they are unpacked at entry, they should be unpacked at exit. If they are packed at entry, they need not be copied when unpacking at exit. This saves unnecessary operations.

News snippet

Save operations by removing unnecessary copying in unpack statements in hamiltonian and exponential.

Checklist

• I have checked that my code follows the Octopus coding standards