v0.2.1: Better support for ProxyStore

The main feature addition is the ability to use different backends for different tasks.

What's Changed

• Simplify Parsl task server by @WardLT in #44
• Refactor result class by @WardLT in #45
• ProxyStore Improvements by @gpauloski in #46

Full Changelog: v0.2.0...v0.2.1