Add tests for gmx_genion
In order to migrate the existing gromacs tools to the TrajectoryAnalysis framework, it is necessary to have proper unit tests. One challenge for putting of some of the tools under test is that they require a tpr as input, which means that the test needs to actually construct the tpr as part of the test construction. One such tool is gmx_genion. In order to construct a tpr that is large enough to actually add ions to, spc216.gro in gmxpreprocess/tests/ can be used, but it needs to be modified to have a large enough box size so that grompp does not complain about the box being to small for rlist. A .top file is also needed, though this is trivial to create and could even be part of test construction. In order not to have two copies it might be a good idea to move gmx_genion to gmxpreprocess/ since anyway it seems like a necessary preprocessing step.
Related to #2727