Align the build system back to the one in opm-core

This changeset makes finitevolume.cc an example, and then remove the not_unit_tests target from the build system. This is not done with a light heart, but I really don't want to fork the build system as there are numerous patches from opm-core that is pending to be applied on top of it. Therefore, I ask:

@blattms:
What is the use case of building `not_unit_tests`? Are there actually any testing value in these executables, or is the purpose to check if they still compile and link? Is it useful enough to have a `not_unit_tests` (renamed to something more intuitive) in **all** the OPM modules? Is it feasible to promote them to tests which always return true (so the test is really to see if they compile)?