Fix Shorting functionality

[mikeejazmines]

Description

I noticed that shorting did not completely work yet. I fixed the shorting logic so that it follows the following steps:

If buy signal:

• It will first check what is the current position size (if there is an open short)
• calculate the size it can buy with the available cash
• combine the two to get the net position size to open

If sell signal:

• It will first check what is the current position size (if there is an open long)
• calculate the size it can buy with the available cash
• combine the two to get the net position size to open

Notebook under examples is labelled as 2022-03-01-backtest_crypto_short

Testing worked as expected:

Long only orders:

Long and short orders:

General results:

Checklist

• [x] I am making a pull request from a branch other than master
• [x] I have read the CONTRIBUTING.md
• [x] I have added/edited documentation in a relevant docs/docusaurus markdown file