Edge-hanging corners in mesh

[hannesbrandt]

p4est_mesh: add corner neighbors across coarse edges

Following up on issue #244 and the test proposed by @scottaiton in #235.
We introduce a p4est_mesh_params_t that contains all mesh-creation parameters with the possibility to add further parameters in the future. The p4est_mesh_t now contains a p4est_mesh_params_t instance to store the parameters it was created with.
In 3D, we add a new edgehanging_corners flag to the set of parameters. When set to true, quadrants that are corner neighbors across a coarse edge are stored in the same format as all other corner-neighbor relations of the mesh. If set to false, the mesh is created as usual.
Furthermore, we add p4est_mesh_new_params which creates a mesh according to a passed p4est_mesh_params_t instance. A default parameter instance can be created with p4est_mesh_params_new and destroyed with p4est_mesh_params_destroy.
Apart from this, we remove a few unreachable lines of code in mesh_iter_edge and add some documentation in p4est_mesh.h. Especially, p4est_mesh_get_neighbors is now marked as outdated.