Allow Users to Pass Data Instead of Alpaca

[KendelChopp]

Allow users to pass in data instead of requesting it through passed in Alpaca. This would benefit us in 2 ways:

1. Most importantly, users would not have to pass in their authenticated Alpaca object into the package. Instead, they can fetch the data themselves then pass it in, and we never touch Alpaca.

2. Allow users to retrieve data from sources outside of Alpaca to use for testing.

For this, we won't worry so much about verifying the schema of the data, but it is something we can think about down the line. Additionally, we should update the README to show users how to do this.

To accomplish this will likely require modifying:

• Backtest constructor to accept Alpaca OR bar data
• Websocket constructor to accept the loaded data
• Websocket loadData to use the already loaded data rather than hitting the Alpaca object
• README to reflect these changes.