Coinbase OHLCV Data Extraction

I need a clean, well-documented script that pulls 1-hour OHLCV candles for Bitcoin (BTC) and Ethereum (ETH) from Coinbase and saves them locally in a reusable format. Python is my preferred language and ccxt or the native Coinbase API are both fine, as long as the code is straightforward to maintain.

What the script must do
• Accept a start and end date (or default to “everything available”) and fetch all 1-hour candles in that range.
• Gracefully respect Coinbase rate limits and automatically retry on transient errors.
• Output each run to a CSV (one file per asset is OK) and optionally append to an existing file so the data set can be updated on a schedule.

Quality checks
• Timestamps must be ISO-8601 and strictly consecutive with no missing hours—report any gaps you cannot fill.
• Open, High, Low, Close, and Volume should be numeric and match values returned directly by the API.
• The code should run from the command line with a single command, e.g. `python collect_ohlcv.py --symbol BTC --start 2020-01-01`.
• Include a short README explaining setup, required libraries, and an example cron entry for daily updates.

Deliver the source code, README, and a sample output file so I can confirm formatting before final acceptance.