MRG: fix ligands; move `is_present` to be an interaction

[ctb]

Move is_present over to be an interaction (that is allowed in multiple tissues); allow empty states => inactive.

Bumps to v0.3.2.

Release checklist:

checklists

validation:

• [x] tests passed
• [x] sekret notebook succeeded
• [x] example notebooks all ran without error

technical:

• [x] bumped version in pyproject
• [ ] released on github
• [ ] released to pypi