Fix non-unit tests so that they compile again

Account for header, namespace and API changes.
Graduate these tests to unit tests

Although they don't use Boost::UnitTest, they can at least pass, so we
can use them to detect simple compilation and runtime errors, although
we miss the semantic check.

(If you have time, please make them proper unit tests)
Convert Cartesian grid test to use Boost::UnitTest
Cartesian grid test is now a proper unit test
Convert cubic interpolator to use Boost::UnitTest
Convert testing of metric units to use Boost::UnitTest
Graduate testing of metric unit to a proper unit test

Notice the confusing naming.
Add warning that this directory is not actively maintained.
Rename test itself to not conflict with local variable
Indent consistently with whitespace instead of tab
Merge pull request #294 from rolk/294_unittests

Fix non-unit tests so they compile again