Consider using `quantities`

[llucax]

So far the client doesn't depend on quantities and use plain floats.

We could add quantities, but if we do, we probably want to use it everywhere, including streaming data.

This adds overhead though, as creating new instances of non built-in types in Python is very slow compared to working with plain floats, and maybe it is better to keep the client simple and performant as it should be mostly used only for low-level stuff.

Using quantities also means maintaining a map of Metric -> Quantity, for example for streaming data and bounds.

Originally posted by @llucax in #165 (comment)