Python wrapper: default con'tors

[akuesters]

Check necessity of default constructors and their documentation string.

• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/context.cpp#L97
• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/domain_decomposition.cpp#L57
• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/identifiers.cpp#L24
• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/mpi.cpp#L103
• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/recipe.cpp#L205
• https://github.com/arbor-sim/arbor/blob/66b04c127b20783ad29799d3d7aea49e22df69ec/python/schedule.cpp#L178

[akuesters]

Also check if non-default constructors are missing:

https://github.com/arbor-sim/arbor/blob/master/python/cells.cpp#L217

[thorstenhater]

• cells.cpp: no, fine.
• recipe.cpp: should have a default ctor, since we call __super__
• schedule.cpp: Kind of useless, but ok.
• mpi.cpp: defaults to MPI_COMM_WORLD
• identifiers.cpp: cell_member_type should not default to (0, 0). It doesn't in the current master
• context.cpp: default context is fine.