Nexus: update how wavefunction is taken by hamiltonian following 5825 input change

[ye-luo]

Following the input changes introduced in
#5825

Existing files should not break unless errors were detected by the new change.

I think for nexus, just don't set the wavefunction attribute unless the simulation is CSVMC.

Not sure what principal nexus follows to generate an QMCPACK input. My recommendation is to write minimal input which is usually compatible with both old and new input scheme.

[jtkrogel]

Whatever the update to Nexus, it needs guaranteed backward and forward compatibility.

[ye-luo]

Nexus as a workflow tool has its own set of rules about how to generate input files. If it sticks to the old input, nothing breaks. The input files are just verbose.

[ye-luo]

Nexus currently generates old style

      <hamiltonian name="h0" type="generic" target="e">
         <pairpot type="coulomb" name="ElecElec" source="e" target="e"/>
         <pairpot type="coulomb" name="IonIon" source="ion0" target="ion0"/>
         <pairpot type="pseudo" name="PseudoPot" source="ion0" wavefunction="psi0" format="xml">
            <pseudo elementType="O" href="O.BFD.xml"/>
         </pairpot>
      </hamiltonian>