Improve backtesting speed

[MDUYN]

Support backtesting for a single buy and sell function, where the entire context of the algorithm is not needed.

create_buy_signal(data) -> bool:
.....
create_sell_signal(data) -> bool:
.....

report = app.run_backtest(buy_signal_function=create_buy_signal, sell_signal_function=create_sell_signal)

This allows for faster backtesting and evaluation of your signal generators without the need to run an entire algorithm.
Running the entire algorithm can be useful for more real world backtesting scenarios, but for experimentation you really just want
to quickly evaluate your buy and sell signal functions.