Introduce new type for featurizing Atoms objects

[thazhemadam]

Presently, the featurization in AtomGraph is present for effective serialization support. However, this is not without its issues (see #85). However, instead of adding a new featurization type, I propose adding a new type that encapsulates the data that would be serialized otherwise, and use this for serialization purposes (and more). This new type would essentially contain an AbstractAtoms object, AbstractFeaturization object, and an encoded_features field. The features stored in encoded_features can always be decoded using the AbstractFeaturization associated.

Some of the pros that this approach offers -

• Atoms object becomes more atomic
• decreased coupling between Atoms and Featurization objects
• completely independent, and customisable serialization (could be specialized for different types if required too, but I think it should be fairly generic)
• no need to add a new pseudo featurization type that does nothing
• no need to additionally set the field to nothing once a convolution op gets performed on an Atoms object
• relaxes the constraint on new types of AbstractAtom, as they won't need to have the featurization field
• Present serialization logic as present in the constructor for AtomGraph can be simplified. This would also apply to other types of AbstractAtoms types (as they're developed).

[codecov-commenter]

Codecov Report

Merging #91 (99dd901) into main (6123ed5) will increase coverage by 2.27%. The diff coverage is 65.51%.

:exclamation: Current head 99dd901 differs from pull request most recent head bdd35ec. Consider uploading reports for the commit bdd35ec to get more accurate results

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   83.01%   85.28%   +2.27%     
==========================================
  Files          11       12       +1     
  Lines         312      299      -13     
==========================================
- Hits          259      255       -4     
+ Misses         53       44       -9     

Impacted Files	Coverage Δ
src/ChemistryFeaturization.jl	66.66% <0.00%> (-33.34%)	:arrow_down:
src/features/features.jl	66.66% <0.00%> (-33.34%)	:arrow_down:
src/atoms/atomgraph.jl	72.58% <61.11%> (+10.24%)	:arrow_up:
src/featurizedatoms.jl	83.33% <83.33%> (ø)
src/featurizations/featurizations.jl	100.00% <100.00%> (ø)
src/featurizations/graphnodefeaturization.jl	90.69% <100.00%> (-1.31%)	:arrow_down:

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