Annoying, cannot rely on equivalence of Grid ComplexD adn Eigen Complex type on GPU.

Solve with ComplexD typecasts but must be a better way

Grid/algorithms/iterative/PrecGeneralisedConjugateResidual.h

@@ -41,6 +41,8 @@ NAMESPACE_BEGIN(Grid);
 template<class Field>
 class PrecGeneralisedConjugateResidual : public OperatorFunction<Field> {
 public:                                                
+  using OperatorFunction<Field>::operator();
+
   RealD   Tolerance;
   Integer MaxIterations;
   int verbose;