Feature/add interface library

This follows a pattern we want to establish in our cmake buildsystem. Every module should have a library even if it does only acts as an interface for dependencies. This MR adds the interface library Dune::FoamGrid.

CMakeLists.txt

@@ -17,7 +17,9 @@ include(DuneMacros)
 
 # start a dune project with information from dune.module
 dune_project()
-dune_enable_all_packages()
+dune_add_library(dunefoamgrid INTERFACE
+  EXPORT_NAME FoamGrid 
+  LINK_LIBRARIES Dune::Grid)
 
 add_subdirectory(dune)
 add_subdirectory(doc)
@@ -29,9 +31,9 @@ if(DUNE_ENABLE_PYTHONBINDINGS)
 endif()
 
 # finalize the dune project, e.g. generating config.h etc.
-finalize_dune_project(GENERATE_CONFIG_H_CMAKE)
+finalize_dune_project()
 
 # set policy for install accepting generator expressions
-if ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+if (POLICY CMP0087)
   cmake_policy(SET CMP0087 NEW)
-endif ("${CMAKE_VERSION}" VERSION_GREATER 3.14)
+endif ()