rename mesh.pyx -> cmesh.pyx
new C mesh data structures

- new mesh.c, mesh.h
- new MeshGeometry, MeshConnectivity, ConnIter, MeshTopology, Mesh, Region
- mesh construction, printing, connectivity iterator
- stubs for connectivity computation algorithms
new CConnectivity, CMesh - cython wrappers of C mesh structures

- construct C Mesh from Python Mesh
setup.py: add mesh.c to cmesh sources
update mesh_print(), mesh_set_coors()
new EntityIter + entity iterator functions
fix indexing of connectivities
fix conn_print(), new conn_iter_print_current()
allocate connectivity arrays in C

- new conn_alloc(), conn_free()
- update CMesh._set_conn(), .from_mesh()
new CConnectivity.cprint()
new mesh_free_connectivity()
new CMesh.setup_connectivity(), .free_connectivity()
clean up mesh_setup_connectivity()
implement mesh_transpose()
unify mesh entity and connectivity iterators

- new MeshEntity, MeshEntityIterator + related functions
- remove ConnIter, EntityIter + related functions
- update conn_print(), mesh_transpose()
- new conn_set_to_free()
fix error check in mesh_transpose()
new functions for getting entities incident to given entity

- new Indices, new me_get_incident(), me_get_incident2(), contains()
implement mesh_intersect()
move typedefs to avoid wrong forward declaration
new create_geometry_elements()