force bagel.tsv to be loaded as string type to avoid unwanted type casting

[surchs]

We should force pandas to load bagel.tsv and cast all columns to type str - I don't think this should break anything, but the tests should tell us.

This addresses the issue of pandas implicitly casting values to non-string types (e.g. "True" and "False" being cast to bool), breaking things in unexpected and silent ways.