[Feature] Add Molpro as a backend via pymolpro

[JGrantHill]

Enables HF energies (HF, RHF and UHF) with a Molpro backend using the pymolpro interface. Additional methods and properties to be added.

Somewhat accidentally also updates the multiple molecule example to use a finer DFT integration grid.

[codecov]

Codecov Report

Attention: Patch coverage is 2.22222% with 88 lines in your changes missing coverage. Please review.

Project coverage is 51.60%. Comparing base (ec76df0) to head (ef412ed).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
basisopt/wrappers/molpro.py	0.00%	83 Missing ⚠️
basisopt/api.py	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
- Coverage   52.71%   51.60%   -1.11%     
==========================================
  Files          47       48       +1     
  Lines        2963     3110     +147     
==========================================
+ Hits         1562     1605      +43     
- Misses       1401     1505     +104     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[JGrantHill]

Should note, this now requires pymolpro == 1.3.19 (and presumably later versions)

[robashaw]

I'm back after an extended hiatus! And will be merging the three open PRs in quick succession. I can't test this fully as I don't have molpro anymore, but the code looks good.

Only comment is that we shouldn't commit "poetry.lock" files unless we've published a new package version