Notes on 3d Gravity Wave test case

Added notes on 3d Gravity Wave test case (see http://arxiv.org/abs/1411.2940), which should be used as a testcase for a three dimensional solver.
Revert "Notes on 3d Gravity Wave test case"

This reverts commit e392b3980e3c075b7568ea69df96cc90b0e37b78.