Add the TWOSIDES dataset with molecular features

[benedekrozemberczki]

• The dataset has to be mapped to some common identifier system.
• We need the molecular structures.
• We also need negative samples.

[cthoyt]

see https://github.com/kexinhuang12345/data_process/blob/c1ee3e036c9502501b7e8c4a590de8533985e328/data_process/DDI_utils.py