Restructure package

[obackhouse]

• Restructures the package into submodules
• Tidies up some of the inheritance
• Makes the package mypy compliant (now checked in CI)
• Docstrings are now Google format since we don't need them to hint types
• Drops support for 3.9, adds 3.12
• Fixes Density fitting fails when not incore #48 and fixes AttributeError: 'str' object has no attribute 'flags #57

[codecov]

Codecov Report

Attention: Patch coverage is 79.73752% with 633 lines in your changes missing coverage. Please review.

Project coverage is 98.39%. Comparing base (cbcec51) to head (91c716c).

Files	Patch %	Lines
ebcc/eom/reom.py	20.52%	119 Missing and 1 partial ⚠️
ebcc/cc/rebcc.py	78.14%	57 Missing and 16 partials ⚠️
ebcc/cc/base.py	84.78%	57 Missing and 13 partials ⚠️
ebcc/opt/gbrueckner.py	23.43%	48 Missing and 1 partial ⚠️
ebcc/cc/gebcc.py	91.17%	20 Missing and 22 partials ⚠️
ebcc/ham/space.py	77.30%	30 Missing and 7 partials ⚠️
ebcc/eom/base.py	86.52%	21 Missing and 5 partials ⚠️
ebcc/cc/uebcc.py	88.02%	13 Missing and 7 partials ⚠️
ebcc/eom/geom.py	86.75%	8 Missing and 12 partials ⚠️
ebcc/ham/eris.py	68.25%	14 Missing and 6 partials ⚠️
... and 17 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   98.77%   98.39%   -0.38%     
==========================================
  Files          78       88      +10     
  Lines       97586    98309     +723     
  Branches      834      903      +69     
==========================================
+ Hits        96388    96733     +345     
- Misses       1055     1395     +340     
- Partials      143      181      +38     

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