This commit completes the migration to pysheds, and represents a minimal working version of the pfdf package. Key updates include: * The Segments class has been refactored for pysheds * Segments class now calculates earth system variables needed to run probability, volume, and combined hazard models * The staley2017 module now solves for both probability and accumulation * Simplified the calculation of earth system variables in staley2017 * Added a utility module to convert between various types of slope metrics