Dipolar tests

[RudolfWeeber]

fixes #1569 closes #4046

This is based on #4046, minus the last commit.

Description of changes:

• Switch tests for dipolar interactions with open boundary conditions from comparing two methods against each other to comparing individual methods against reference data
• Use prefactor != 1 in dipolar 2d and 3d periodic tests
• Block DipolarDirectSumWithReplicaCpu from n_replica=0 and periodic bc, as it does not apply minimum image convention. Introducing minimum image convention there is not worth it, because that is provided by DipolarDirectSumCpu. The two methods should be merged anyway.

[jngrad]

LGTM