fix(controller): handle missing FlowStatsfile.csv via env-configurable path; add troubleshooting docs

[DivInstance]

Summary

• Make training dataset path configurable via `FLOW_STATS_FILE`.
• If dataset file is missing, log a clear error and safely skip training (prevents `FileNotFoundError`).
• Added README troubleshooting section documenting how to provide the dataset or set the env var.

Motivation

Users frequently hit `FileNotFoundError` when `FlowStatsfile.csv` is not present in the working directory. This change improves robustness and developer experience.

Notes

• Backward-compatible default (`FlowStatsfile.csv` in CWD).
• Addresses: Fixes FileNotFoundError: [Errno 2] No such file or directory: 'FlowStatsfile.csv' #13; improves docs relevance for [Documentation] Improve clarity and organization of documentation #9.