Merge branch 'parallel_mesh_assignment' into 'master'

doc/RTD/source/ParameterFiles/parameter_description.rst

examples/parameter_example.yml

src/gravity_properties.c

src/gravity_properties.h

src/mesh_gravity.c

src/mesh_gravity.h

src/mesh_gravity_mpi.c

src/mesh_gravity_mpi.h

src/mesh_gravity_patch.c

src/mesh_gravity_patch.h