Split between unit and regression tests

[maxnoe]

To make running the tests faster and easier to see breakage,
we should split the test suite between short running unit tests and long runnnig physics results regression tests.

For this, we could also use travis staged builds. So that e.g. the regression tests are only run after the unit tests were successful.

[maxnoe]

See https://docs.travis-ci.com/user/build-stages/ with that exact example (unit vs. integration tests)