Adds an initial GitHub Actions workflow

[rafmudaf]

Complete this sentence
THIS PULL REQUEST IS READY TO MERGE

Feature or improvement description
This pull request adds a simple GitHub Actions workflow. The workflow is primarily run on Docker from a base image at rafmudaf/openfast-ubuntu. This image has all of the dependencies preinstalled and has OpenFAST compiled at commit hash de5b749 (@andrew-platt's pull request on 10/23/19). For every new commit to dev, GitHub Actions will only recompile the new changes saving quite a bit of time over TravisCI. Then, all of the BeamDyn-specific tests and a subset of the OpenFAST regression tests are run which is also an improvement over TravisCI where none of the OpenFAST regression tests are run due to time constraints.

Related issue, if one exists
This pull request doesn't solve the issue, but a related one is at #52.

Impacted areas of the software
.github/ directory and continuous integration.

Additional supporting information
This is an initial implementation of GitHub Actions. Next steps are:

1. Automatically update the Docker image after each commit to dev
2. Enable all of the regression tests
3. Run the GitHub Actions for the following events

• Any commit on any branch on openfast/openfast
• A pull request
• A tag

3. On merge to master

• Automatically tag
• Automatically create a release
• Add binaries to the release
• Create a new Docker image and upload to Dockerhub

Test results, if applicable

[rafmudaf]

Furthermore, either I'm missing something or TravisCI is flakey. It didn't run for this pull request!

[andrew-platt]

Does TravisCI run if there are no code changes?

[rafmudaf]

I think as far as TravisCI is concerned, this is a code change. It doesn't have a sense of what is or isn't source code. On my TravisCI dashboard, it also shows that I'm not a part of the NREL or OpenFAST organizations, so I can't manually trigger a build.