Change API for getting article dataframes and add pretoken analysis

[cthoyt]

This PR does the following

1. renames biolexica.literature.get_pubmed_dataframe to biolexica.literature.get_article_dataframe_from_pubmeds (breaking change)
2. makes batching part of this process from default
3. adds biolexica.literature.get_article_dataframe_from_search
4. adds pre-word analysis and example

[codecov]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (3b472cd) 46.38% compared to head (56abc10) 44.39%.

Files	Patch %	Lines
src/biolexica/literature/analyze.py	0.00%	21 Missing ⚠️
src/biolexica/literature/search.py	50.00%	4 Missing ⚠️
src/biolexica/literature/retrieve.py	86.36%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   46.38%   44.39%   -2.00%     
==========================================
  Files          12       12              
  Lines         373      410      +37     
  Branches       75       70       -5     
==========================================
+ Hits          173      182       +9     
- Misses        193      219      +26     
- Partials        7        9       +2     

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