Update chemical descriptions

[christinehc]

Tasks:

• @christinehc Update and fill in chemical descriptions: ChemicalDescriptions.xlsx
• @christinehc Combine with chemical ID mapping table to simplify pipeline: chemicalIdMapping.csv
• @christinehc Integrate into pipeline

[christinehc]

Progress update:

• Chemical descriptions filled in via PubChem API, but 995 chemicals still lack descriptions.
• Created a combined ID mapping and descriptions table. NOTE: Where on Figshare should we upload these files?
• Pipeline integration in progress

[David-Degnan]

• @bartonmike Determining chemicals with no information, and whether we need to add more chemical descriptors
• @ginamany Add anything missing

[christinehc]

@ginamany