Add "source" variable to CANDO compounds mapping

[zfalls]

This will be an additional variable that will be present in the mapping file and will be processed on init. Each compound will have an associated source (public db, design protocol, etc.) from which it was extracted.

• [ ] Add "source" column to compound mapping file
• [ ] Add new variable to Compound class that will be populated by "source" during CANDO init
• [ ] Optional: Output "source" in canpredict and other functions