Add an assertation

Note: This assumes that any vector used to construct a BasisGridFunction
already has the proper size. If you want to be able to write

  Vector vec; // unitialised
  BasisGridFunction func(vec);
  initialize(vec);

for some reason, then please revert this change and let me know.

dune/fufem/functions/basisgridfunction.hh

@@ -217,7 +217,9 @@ class BasisGridFunction :
             BaseType(basis.getGridView()),
             basis_(basis),
             coefficients_(coefficients)
-        {}
+        {
+          assert(basis_.size() == coefficients_.size());
+        }