Integral / cumulant export APIs

[brianz98]

Description

This PR introduces various new APIs that allows the user to save various MR-LDSRG(2) quantities to disk, including

• The full similarity transformed Hamiltonian (as a dictionary of NumPy arrays), for SA and SI variants
• The full similarity transformed Hamiltonian normal ordered w.r.t. the physical vacuum, for SA and SI variants
• Up to 4-body cumulants, for SI variant (4-RDM/cumulant code is new)
• The full similarity transformed dipole integrals (0-2 body)

Checklist

• Added/updated tests of new features and included a reference output.ref file
• Removed comments in code and input files
• Documented source code
• Checked for redundant headers
• Checked for consistency in the formatting of the output file
• Documented new features in the manual
• Ready to go!