macOS and Windows environments on travis tests

[james-large]

For completeness, these test envs should be added too, however they would be non-trivial.

Versioning between operating systems is of course one issue. See here, as of writing macOS supports up to tf 2.0, but not 2.1 (on cpu, that is)

Windows should be able to support any version, but windows envs on travis are less feature-rich. It should be possible in principle, https://docs.travis-ci.com/user/reference/windows/

Alternatively, docker/equivalent environments could be setup and used within the travis tests to alleviate some of these problems. #27

Investigation into other open-source projects and how they do multi-os tests would be good

[mloning]

Closed by #50