Move all implementations of functions to a 'functions' subdirectory

CHANGELOG.md

 # Master
 
+- All implementations of functions have been move to a `functions` subdirectory.
+
 - The `LocalDensity` class now has a template parameter `ElementOrIntersection`,
   which replaces the parameter `Position`.  As the name says, this parameter
   has to be a grid element, or a grid intersection.  As it turned out,

dune/gfe/CMakeLists.txt

@@ -4,19 +4,13 @@ install(FILES
         cosseratstrain.hh
         cosseratvtkreader.hh
         cosseratvtkwriter.hh
-        embeddedglobalgfefunction.hh
         filereader.hh
         geodesicdifference.hh
         geodesicfefunctionadaptor.hh
         gfedifferencenormsquared.hh
-        globalgfefunction.hh
         gramschmidtsolver.hh
-        interpolationderivatives.hh
         linearalgebra.hh
-        localgeodesicfefunction.hh
         localgfetestfunctionbasis.hh
-        localprojectedfefunction.hh
-        localquickanddirtyfefunction.hh
         localtangentfefunction.hh
         maxnormtrustregion.hh
         mixedriemanniantrsolver.cc
@@ -42,3 +36,4 @@ install(FILES
 add_subdirectory("spaces")
 add_subdirectory("assemblers")
 add_subdirectory("densities")
+add_subdirectory("functions")

dune/gfe/functions/CMakeLists.txt

+install(FILES
+        embeddedglobalgfefunction.hh
+        globalgfefunction.hh
+        interpolationderivatives.hh
+        localgeodesicfefunction.hh
+        localprojectedfefunction.hh
+        localquickanddirtyfefunction.hh
+        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/gfe/functions)