Set index if specified in output schema

[donaldcampbelljr]

Closes #105

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0ea22d4) 62.73% compared to head (9bf6bca) 62.69%.

Files	Patch %	Lines
pipestat/backends/db_backend/db_parsed_schema.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #141      +/-   ##
==========================================
- Coverage   62.73%   62.69%   -0.04%     
==========================================
  Files          15       15              
  Lines        2321     2327       +6     
==========================================
+ Hits         1456     1459       +3     
- Misses        865      868       +3     

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[khoroshevskyi]

LGTM

Could you please add docs about indexing

[donaldcampbelljr]

Great. I've added the requested docs.