Add pubchem2500.sdf.gz

This adds a new file containing the first 2500 molecules
from PubChem in the SDF format.

Change-Id: Ib95d5baa16c919a386e589d3cf1f164379d609b9
Merge topic 'pubchem-2500' into master

9dfba8f9 Add pubchem2500.sdf.gz