✨ Enhance default dataframe schema

[ff137]

We now by default expect OHLCV (open-high-low-close-volume) values to be float32's, for lower memory usage by default.

To work with 💩 -coin data, measured in satoshis, you may want to request float64's for higher precision, but float32 (7-decimal precision) is more than good enough for most scenarios.

If you are working with data with no decimals, you may want to use int32 dtypes

We now also set the index of the OHLC dataframe to represent the datetime of the timestamp value.