Fix initial cartesian units

[timostrunk]

Description

This PR fixes #3147 . Basically the psithon way of defining molecules converted _initial_cartesian on parsing.
When adding an external potential this was used as
molecule.set_geometry(clone_molecule._initial_cartesian).
set_geometry applies the conversion again.

The two python ways I found, which set initial_cartesian did not do the conversion leading to incorrect geometries, when using external_potential and molecules defined as unit=angstrom.

User API & Changelog headlines

• Fixes Using external potential in python broken in case of Angstrom units #3147: Parsing molecules with angstrom units and using external_potential from python was broken

Dev notes & details

• Fixes Using external potential in python broken in case of Angstrom units #3147: Parsing molecules from psithon and python left mol._initial_cartesian in a different unit. This aligns the units. Downstream this had the effect of breaking using molecules with angstrom units with an external_potential from python

Questions

• Question1

Checklist

• Tests added for any new features
• All or relevant fraction of full tests run

Status

• Ready for review
• Ready for merge

[timostrunk]

I ran pytest quick tests, all pytest still running

[timostrunk]

All tests ran through except for:
FAILED tests/fsapt1/test_input.py::test_fsapt1 - AssertionError: Traceback (most recent call last):
with the message:
E !----------------------------------------------------------------------------------!
E ! !
E ! Fatal Error: SCF_SUBTYPE=INCORE was specified, but there is not enough memory to !
E ! do in-core! Increase the amount of memory allocated to Psi4 or allow for !
E ! out-of-core to be used. !
E ! Error occurred in file: /home/conda/feedstock_root/build_artifacts/psi4nm_171103 !
E ! 8779956/work/psi4/src/psi4/lib3index/dfhelper.cc on line: 261 !
E ! The most recent 5 function calls were: !
E ! psi::PsiException::PsiException(std::__cxx11::basic_string<char, !
E ! std::char_traits, std::allocator >, char const*, int) !
E ! psi::DFHelper::initialize() !
E ! !
E !----------------------------------------------------------------------------------!

I think this is independent of this PR and the PR would be good to go

[jturney]

Seems pretty straightforward. Looks good!