Add some unit tests for surface area calculation

There are a few real unit tests that compare the results against
analytical calculation for trivial cases, but the rest of the tests are
more of regression tests, designed to catch changes in the output.
I have not actually checked that the reference values for the more
complex values are correct.

Put the tests in surfacearea.cpp instead of nsc.cpp and renamed the
implementation to match in preparation for future cleanup.

The volume computation with PBC is not tested, since it is broken.

Change-Id: I18e3baa899b1c10fec0c8b43d8dd76a357297005