Release smif v1.2.0

[tomalrussell]

Release checklist:

• [x] create a release branch
• [x] update CHANGELOG with notes on features and fixes since last release.
• [x] update AUTHORS if necessary
• [x] create and push an annotated tag
• [x] open a pull request
• [x] wait (~15 minutes) for tests to pass
• [x] check that deploy stage ran as expected, new version available on https://pypi.org/project/smif/#history
• [x] create a GitHub release for the tag
• [x] wait (~1 day) for conda bot to open PR on https://github.com/conda-forge/smif-feedstock
• [x] check dependencies/versions/lints and merge to release conda package

See more notes in https://smif.readthedocs.io/en/latest/developers.html#releases