Allow fractional units

[GitPeterJ]

Enhancement description

I ran into the same problem as in #134 and after reading a bit about it I wondered, what is the reason why this is not allowed? I understand some brokers do not allow it, but some do, so it should be up to the user to decide whether to put this restriction in place.
Can you explain why you did this? The current solution seems a bit.. cumbersome.

[Zirafnik]

I think this was added in the last update: https://kernc.github.io/backtesting.py/doc/backtesting/lib.html#backtesting.lib.FractionalBacktest

Change Log: https://github.com/kernc/backtesting.py/blob/master/CHANGELOG.md#062

It basically just uses a trick by dividing the prices with the minimum allowed trade size (e.g. for BTC => one 100 millionth of BTC == 0.00000001 BTC).

I do think the docs are somewhat unclear & confusing... I think the keyword argument being named "satoshi" is confusing for the average user. I also strongly believe that documentation for FractionalBacktest should be located next to docs for Backtest. It took me way too long to realize this is a backtesting class ("duh!") and not a Strategy class.