Refactor hsolver remove psi dimensions

[Cstandardlib]

Linked Issue

#4404

What's changed?

• Most inner access to psi dimensions (by calls to member funcions) has been removed.
• Unnecessary class member of DiagoDavid has been replaced by local variables where needed.

Unit Tests and/or Case Tests for my changes

• Corresponding unit tests are modified to fit new function signature.

Any changes of core modules?

• Remove deprecated psi.get_k_first() logic

The psi.get_k_first() function is no longer needed as the k_first=false branch does not contribute to the solution of the Kohn-Sham equations. Remove the associated code to streamline the solver.

Pending further work

• psi.get_ngk(0) and psi.get_pointer() are still referenced inside DiagoDavid, to construct basis set. They need to be replaced by pointers to data field.
• The basis set variable, of type psi, can only be removed after the above steps are completed.