Package usability issues

[zyteka]

Closing issues

Closes #134

Description

Fixes usability issues with the package version of sibyl-api.

• prepare-db and prepare-sample-db should now work when run from the package without a corresponding dbdata directory. Data for the sample db is loaded from pyreal
• invoke prepare-db and invoke prepare-sample-db can now be used by those using the full repo with dbdata
• xgboost is no longer a dependency
• Preprocessing functions are now easier to access

Test Plan

Manually tested all new functions on two datasets