[cmake] add macro dune_add_pybind11_module

Building Python modules through Pybind11 is a recurring task in our
Python bindings. To reduce maintainance in the build system, we introduce
a CMake function "dune_add_pybind11_module", which is semantically inspired
by the "dune_add_test" function.

CMakeLists.txt

@@ -8,5 +8,4 @@ add_python_targets(common
   project
 )
 
-add_library(_common SHARED _common.cc)
-set_target_properties(_common PROPERTIES PREFIX "")
+dune_add_pybind11_module(NAME _common)