Added Euler-Euler coupling test

- moved the test to solver/fv/euler/tests.
- Euler-Euler coupling condition works again.
- moved make_cube helper function into
  geometry/implicit.hpp geometry::make_cube.
- make style-check passes now for all solver/fv files (see #52).
- removed heat-euler test because:
  - its pointless, and
  - its already under revision control, so if required one
  can find it here.