Ewald: Add tests for direct sums with dipole

The direct sum needs to be corrected if the unit cell contains a dipole moment.
Ewald: make direct_sum() working for dipoles

Previous the direct_sum() was only working if the unit cell dipole moment is
zero. Now it works for non-zero moments as well.
Merge pull request #46 from certik/ewald

Ewald: direct_sum() for dipole moment