TreeFrog: detect correct filename if data file given instead

[brittonsmith]

PR Summary

This resolves Issue #98 by introducing a function to get the forest stats filename from a given data filename.

PR Checklist

• [ ] Code passes tests.
• [ ] New features are documented with docstrings and narrative docs.
• [x] Tests added for fixed bugs or new features.

[manodeep]

Yup - tried it out with the datafile name instead of the foreststats file and it worked just fine.