Feature/8 context retriever

[DivyenduDutta]

Add context retrieval module

Added context retrieval module which returns the context based on most relevant chunks associated with the user input query/prompt
Returns empty string in case no context is found
Returns None in case of an error/exception

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 44.73684% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.14%. Comparing base (c1f66a9) to head (7382353).

Files with missing lines	Patch %	Lines
atlas/core/retriever/context.py	44.73%	19 Missing and 2 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   89.14%   86.14%   -3.00%     
==========================================
  Files          15       16       +1     
  Lines         525      563      +38     
  Branches       49       54       +5     
==========================================
+ Hits          468      485      +17     
- Misses         43       62      +19     
- Partials       14       16       +2     

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[DivyenduDutta]

The retrieved context is in the below format:
[Context 1 | Source: < note title >]
< text >

[Context 2 | Source: < note title >]
< text >