Consistent dunder methods for all complexes

[ffl096]

With this pull request, the dunder methods __contains__, __iter__ and __getitem__ work consistently across all complex types now:

• atom in complex works for any atom now. Previously, depending on the complex class, this was only possible for nodes. Closes Inconsistencies in __contains__ implementation across complex types #273.
• Iterating over a complex now yields all atoms, not just the nodes. This was previously changed to only iterate over nodes (see Inconsistencies in iterating over different complex types #266), but changed now for consistency with other dunder methods. Iterating over the nodes is always possible via the complex.nodes property.
• complex[atom] is implemented consistently to return the user-defined attributes for that atom. Previously, depending on the complex class, this returned the neighbours of the atom. There are other means to access the neighbours, which may also take into account different notions of neighbourhood. Closes Inconsistencies in __getitem__ implementation across complex types #270.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (ced2e43) to head (405c487).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
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+ Coverage   97.36%   97.75%   +0.39%     
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  Files          38       38              
  Lines        3561     3557       -4     
==========================================
+ Hits         3467     3477      +10     
+ Misses         94       80      -14     

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