Implement a bases api and rework abstract types

[akirakyle]

This PR implements the proposal in #26 and thus would close #25 and #26. I've also marked all remaining offending instances of #12 as FIXME for now as I think resolving those should be kept separate from this PR.

The remaining TODOs for this PR are then

• Figure out how to reference pages in https://github.com/qojulia/QuantumOptics.jl-documentation in the docstrings here (related to Docstrings #5 and Create a documentation page #6).
• Address embed does not preserve the basis when a CompositeBasis with just a single site is used #27.
• Update QuantumOpticsBase.jl based on this branch, ensure all tests pass, create CHANGELOG.md, and bump version to v1.
• Update everything else downstream and ensure tests pass (QuantumOptics.jl, QuantumCumulants.jl, WaveguideQED.jl, CollectiveSpins.jl, QuantumSymbolics.jl, QuantumClifford.jl, QuantumSavory.jl, Gabs.jl, anything else?)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 49 lines in your changes missing coverage. Please review.

Project coverage is 17.04%. Comparing base (3d63b38) to head (8a5aa31).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/julia_base.jl	0.00%	12 Missing ⚠️
src/bases.jl	0.00%	9 Missing ⚠️
src/expect_variance.jl	0.00%	8 Missing ⚠️
src/linalg.jl	0.00%	8 Missing ⚠️
src/abstract_types.jl	0.00%	6 Missing ⚠️
src/identityoperator.jl	0.00%	4 Missing ⚠️
src/julia_linalg.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   17.29%   17.04%   -0.26%     
==========================================
  Files          12       12              
  Lines         399      399              
==========================================
- Hits           69       68       -1     
- Misses        330      331       +1     

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[Krastanov]

@apkille , just a heads up that this is happening and it might lightly affect Gabs.jl