Update tskit, relicense under MIT

2021-04-23, Version 0.2.0

Commits

• [d626451f06] Bump version to 0.2.0 (molpopgen)
• [1a43847b37] Refactor and (partially) clean up examples/forward_simulation.rs (#87) (Kevin R. Thornton)
• [f96dd27a23] Change from GPL3+ to MIT license. Closes #81 (#86) (Kevin R. Thornton)
• [e1af9aa0f9] Move forward simulation to examples/. Closes #83 (#85) (Kevin R. Thornton)
• [c82a7f03de] Update to latest tskit (rust). Closes #82 (#84) (molpopgen)
• [7702c3965b] Merge pull request #78 from molpopgen/streamline_simplification_code (Kevin R. Thornton)
• [c03215a3f6] Remove unecessary variables/fns from SegmentOverlapper. (molpopgen)
• [f52517b93f] Merge pull request #77 from molpopgen/tidy_README (Kevin R. Thornton)
• [ad21f4958d] Clean up README. (molpopogen)
• [813547ba05] Move change log to standalone file. (molpopogen)
• [8495b874ba] fix project URL in Cargo.toml (molpopgen)
• [63e11a6ed0] Fix link in docs of TableValidationFlags. (molpopgen)
• [1c281384d1] Fix links in change log to guarge against fork deletion causing (molpopgen)