Restored commented std::isnan check in IterativeSolver

Works fine with g++ 5.5 and nvcc 8.0 in Arch Linux. File a bug report on
Gitlab if you have problems.

src/TNL/Solvers/IterativeSolver_impl.h

@@ -107,12 +107,7 @@ bool IterativeSolver< Real, Index> :: checkNextIteration()
 {
    this->refreshSolverMonitor();
 
-   if(
-#ifndef HAVE_CUDA      
-      std::isnan( this->getResidue() ) ||
-      // TODO: Fix this !!!!
-      // this does not work (at least) with nvcc 8.0 and g++ 5.4
-#endif      
+   if( std::isnan( this->getResidue() ) ||
        this->getIterations() > this->getMaxIterations()  ||
        ( this->getResidue() > this->getDivergenceResidue() && this->getIterations() >= this->getMinIterations() ) ||
        ( this->getResidue() < this->getConvergenceResidue() && this->getIterations() >= this->getMinIterations() ) )