Make StatesPack=yes the default and unify accessing the states

Description

Change the default of StatesPack to yes to improve performance and avoid packing/unpacking the states for every application of the Hamiltonian. There are still a few exceptions that are not yet implemented or tested, for which StatesPack is set to no.

There should be very few cases left in the code that directly access the unpacked version of the states without providing versions for unpacked/packed cases.

This merge request is a more up-to-date extension of !138 (merged) that was started by Xavier.

News snippet

Store states in packed mode as default (StatesPack=yes) to improve performance.

Checklist

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