Merge pull request #167 from EJEP/add_print_capability

Forgot to update Changelog

Author: Emlyn Price

CHANGELOG.md

@@ -5,6 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## [Unreleased]
 
 + Remove deprecated `new_old` and `future_old` functions.
++ Add `__str__` functions to `Timestep`, `Element`, `Day`, `Site`
 
 ## [0.9.8] - 2020-07-03