Fix warnings in debian-9 clang build

Addresses: #3

src/nitschenonlinearpoissonfem.hh

@@ -63,7 +63,7 @@ namespace PPS {
       if (!isdirichlet) return;
 
       // types & dimension
-      const int dim = Intersection::dimension;
+      const int dim = Intersection::Entity::dimension;
       using RF = typename LocalBasis::Traits::RangeFieldType;
 
       // select quadrature rule