[Feature] Adds a well tempered Strategy and an example

[JGrantHill]

Cloned the even tempered Strategy and modified it to produce a well tempered Strategy. Included example file uses Psi4 as the backend to optimise the 4 parameters for a well tempered basis.

TODO:

• Add some information to the docs. (Tutorial doc added)
• Investigate efficiency. The gamma and delta parameters can become negative during the optimisation and while this still produces valid exponents, it is likely to slow things down. Perhaps some regularisation/preconditioning will help.
• Consider adding a well tempered starting guess for a free optimisation (probably a separate PR). (Done)
• Get flake8p to actually run (seems to fall over on my system). (Done)

[codecov]

Codecov Report

Attention: Patch coverage is 37.06294% with 90 lines in your changes missing coverage. Please review.

Project coverage is 50.92%. Comparing base (ec76df0) to head (49b059c).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
basisopt/opt/welltemper.py	22.22%	70 Missing ⚠️
basisopt/basis/atomic.py	35.29%	11 Missing ⚠️
basisopt/data.py	40.00%	6 Missing ⚠️
basisopt/basis/guesses.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   52.71%   50.92%   -1.80%     
==========================================
  Files          47       49       +2     
  Lines        2963     3250     +287     
==========================================
+ Hits         1562     1655      +93     
- Misses       1401     1595     +194     

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[JGrantHill]

I think this is now ready for review. The point about efficiency is one that can probably be solved in the optimisation, rather than a problem with the well-tempered expansion itself.

[robashaw]

As these are all the same, better to make them a constant

[robashaw]

Only one small change needed