feat: saving to CSV file or return CSV string - removing pandas dependency

[davidsbatista]

Related Issues

• fixes Removing pandas as a dependency #101

Proposed Changes:

How did you test it?

• unit tests + manual verification

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
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[davidsbatista]

@ivyleavedtoadflax the idea here is to remove this big dependency, pandas which was also bringing lots of other stuff, like numpy.

Since we're using pandas only to output a DataFrame, I think outputting a .csv is better, which can easily be loaded into a DataFrame if the user wishes so, and then nervaluate becomes self contained no dependencies.

[ivyleavedtoadflax]

Great idea, totally agree. Pandas is huge.