Simplify data output to single directory or filepath

CytoTable may provide a string filepath or nested dictionary data structure contingent on the parameters provided to CytoTable. This issue focuses on simplifying this so that all output is a string which represents a directory or filepath. May relate to #149 which would avoid potential data loss for data provenance.

Start by tracing how CytoTable currently returns a filepath or nested dictionary based on its parameters. Review the related data-provenance concern in issue #149 before deciding the unified output contract. Done means every supported parameter combination returns a string representing a directory or filepath without losing provenance.

Written by the indexing model from the issue text.