Upgrade supply policy rebase curve to a sigmoid curve?

Details and Ampleforth governance forum discussion here

Summary

Change the rebase factor function from a linear curve to an S-shaped curve.

Motivation

The advantages of an Sigmoid curve:

• Can be configured to have different slopes at different points of the curve.

• It has bounds (asymptotes)

  • That makes AMPL supply changes relatively more predictable beyond the current day
  • Limits the potential effect from a bad oracle price. This makes AMPL more resilient to errors and attacks and also increases the feasibility of further oracle decentralization efforts