Move CI to travis.com / github workflows?

[marijanbeg]

This is now done in PR #25. There are two workflows:

1. on-ubuntu-latest.yml: Installs and tests OOMMF on ubuntu-latest - no Docker.
2. in-docker.yml: Installs and tests OOMMF inside Docker container

• Most of the commands are in Makefile.
• .travis.yml is removed.

Things to review/do before closing:

• I am still not happy with Makefile target names and workflow names.
• Should we remove on-ubuntu-latest.yml?
• Remove Travis from Webhooks.

[marijanbeg]

Dockerfile is now based on Ubuntu 20.04.

[fangohr]

Done with PR #25 - thanks Marijan.