Add KLIFS drugs data

[dominiquesydow]

Description

Add drug list to opencadd.databases.klifs via KLIFS API Ligands.drug_list() Part of https://github.com/volkamerlab/opencadd/issues/85

Todos

Notable points that this PR has either accomplished or will accomplish.

• [x] Add Drugs class; add Session.drugs attribute
• [x] Add tests
• [x] Update tutorial with drugs section

Questions

None.

Status

• [x] Ready to go

[codecov-commenter]

Codecov Report

Merging #107 (177273f) into master (1f454e8) will increase coverage by 0.06%. The diff coverage is 88.88%.