Check single tesseroid against many prisms

The results in the GOCE proceedings are confirmed even using
many prisms. However, when turning on division with ratio 4,
the difference is still above e-3! The absolute error never falls
below e-3 even using really high ratios (6-10). So absolute differences
don't tell me much. When using relative differences, a ratio of 2 makes
all differences below 1%.