Improve rdm load save

[marco-2023]

Summary

This PR adds step-awareness to the SHCI RDM save/load functions, enabling proper handling of "variational" and "perturbative" stages in the workflow.

Problem

Previously:

• RDM file naming was inconsistent between the saving and loading steps.
• As a result, SHCI computations with perturbative corrections could fail to locate or load the correct RDM files.
• There was no mechanism to prevent variational RDMs from being overwritten by perturbative ones.

Fixes Introduced

This PR modifies the RDM handling logic to:

• Add a step label ("variational" or "perturbative") to both saving and loading paths.
• Adopt a consistent naming scheme for RDM files in the format prefix/root1-root2-RDMName-step.ext, where step indicates the calculation phase (e.g., variational or perturbative), and ext specifies the file extension (either txt or bkp).
• The workflow was updated for using these functions

Missing

• Apply the same step-aware RDM save/load logic to the ZSHCI module.

[marco-2023]

Here is an example of an affected input.

# Reference determinant
nocc 16
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
end
orbitals o2_ccpvdz_pyscf.fcidump
nroots 1

# Variational calculation parameters
schedule
0 0.0001
end
dE 1e-08
maxiter 20

# Perturbative calculation parameters
epsilon2 0.0001
deterministic
# Additional parameters for output and control
prefix tmp/
DoOneRDM
DoRDM
DoSpinOneRDM
DoSpinRDM