NTR: Add pharmacological relations for small molecules

[cthoyt]

Closes #371

Motivation

is ligand of and has ligand don't say enough about the way that a chemical acts on a receptor. For ChIRO and the Chemical Roles Graph (https://github.com/chemical-roles/chemical-roles), we need more detailed relationships.

What this PR does

This PR creates 7 new properties:

LUID	Name	Parent(s)
0018027	is agonist of	is ligand, is small molecule activator of
0018028	is ligand
0018029	is antagonist of	is ligand, is small molecule inhibitor of
0018030	is inverse agonist of	is ligand
0018031	has agonist	has ligand, has small molecule activator
0018032	has antagonist	has ligand, has small molecule inhibitor
0018033	has inverse agonist	has ligand

Notes:

• this adds both directions of this hierarchy, and to do this, introduces is ligand defined as inverse of has ligand
• doesn't (yet) categorize "is inverse agonist of" under "is small molecule inhibitor of" but that can be done

The inverse tree under "has participants" mirrors the structure set by @pgaudet in #369.