[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

@@ -2,5 +2,4 @@ add_python_targets(istl
   __init__
 )
 
-add_library(_istl SHARED _istl.cc)
-set_target_properties(_istl PROPERTIES PREFIX "")
+dune_add_pybind11_module(NAME _istl CMAKE_GUARD dune-istl_FOUND)