Arkane: add check for iop(2/9=2000) in Gaussian load geometry

[sevyharris]

Motivation or Problem

By default, Gaussian only prints Cartesians in the input orientation for small cases. Add a check for the IOP(2/9=2000) keyword so users don't get tripped up like I did here: #2757 with coordinates that don't match the Hessian.

Description of Changes

I added a check for the IOP(2/9=2000 keyword) when loading the geometry. If the molecule has 14+ atoms (this seems to be the threshold according to my experiments), this will raise a LogError if the keyword hasn't been specified.

Testing

I tested this using one Gaussian logfile that has the IOP and one that doesn't.
conformer_0002_iop.log
conformer_0002_no_iop.log

[sevyharris]

This is currently failing the unit tests because the toluene and benzyl Arkane examples have Gaussian logfiles with more than 14 atoms. I'm rerunning these calculations to see how much it changes the results. If it's a lot, I'll replace the logfiles. If it's not much, that means I've been overstating the error due to this bug and I may not fully understand the problem I thought I was diagnosing.