Add github actions to build, test and deploy OpenMalaria automatically

[acavelan]

Github Actions replace Travis CI.

This new workflow will automatically build, test and release OpenMalaria when the corresponding events are triggered:

• push: build and test
• pull request: build and test
• tag named schema-*: build, test and release (archive uploaded to the release assets)

Tests are performed on all currently available Ubuntu and MacOS versions from Github:

• Ubuntu 20.04
• Ubuntu 18.04
• MacOS 11
• MacOS 10.15

Note that Windows tests and releases will continue to be handled by Appveyor.