Merge branch 'fix-matrixfree-apply' into 'master'

Clear result in matrix-free operator apply()

Closes #10

See merge request jfahl_01/pacxx-projectseminar-2019!13

src/linearsolver.hh

@@ -75,6 +75,7 @@ namespace PPS {
     //! apply operator to x:  \f$ y = A(x) \f$
     void apply (const X& x, Y& y) const override
     {
+      y = 0;
       go_->nonlinear_jacobian_apply(linearizationPoint_, x, y);
     }