PySCF interface for Forte

[brianz98]

Description

This PR creates a new PySCF interface for (MC)SCF reference wfns, integrals, RDMs, etc.

User Notes

• [ ] Features added
• [ ] Changes to compilation (if any)

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!

[shuhangli98]

Two problems

1. It appears that Psi4 has changed, and both FCIDUMP and PYSCF integrals are now broken. Commit 141fcf2 is still fine.
2. Only spin-integrated DSRG can be used. The spin-adaptation code is causing a psi4 segmentation error.

[fevangelista]

@shuhangli98 and @brianz98: Can you please pull the latest version of Forte and adapt this PR to it?

[fevangelista]

There is a conflict with the forte/register_forte_options.py file. I can fix that when merging the PR. On my computer this is what I get when I run the test case:

!----------------------------------------------------------------------------------!
!                                                                                  !
!  Forte: the object (<pyscf.mcscf.mc1step_symm.SymAdaptedCASSCF object at         !
!     0x11d3c8d50>) is not supported. Use CASCI or SCF.                            !
!                                                                                  !
!----------------------------------------------------------------------------------!

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[fevangelista]

This is a very nice addition to the code. Thanks for making this PR!