SLiM-Tree Paper

[ebrintn]

Revisiting SLiM-Tree for publication - list of things that need to happen:

• [x] Update to SLiM 4
• [x] Implement proper counting of dN/dS
• [ ] Revisit user guide
• [ ] Merge dev with master
• [ ] Revisit paper
• [x] Update readme

[ebrintn]

@afarineshp @jasondk please add as needed