Clean up the functions/ directory

It appears that the code located in corepy/functions/ (gridfunctionview.hh and simplegridfunction.hh) is old and in conflict with dune-fempy (according to @andreas.dedner and @martin.nolte ). I am not the author of these files (I think it's you, @martin.nolte , see cf351afd) so I prefer not to touch them.

Although, I investigated on their uses and it appears that they are useful when creating a local grid function to, for example, generate a VTK output (see my finitevolume, finiteelements, parfinitevolume demos). In fact, in this case, the SimpleGridFunction and SimpleLocalFunction classes are instanciated.