Disable regularization per default to avoid unintentional usage.

Thanks to Lasse for pointing that out.

dune/solvers/iterationsteps/blockgssteps.hh

@@ -208,7 +208,9 @@ auto truncateSymmetrically(LinearSolver&& linearSolver) {
 
 namespace LocalSolverRegularizer {
 
-static constexpr double defaultDiagRegularizeParameter = 1e-10;
+// a suitable value for defaultDiagRagularization could be 1e-10
+// nevertheless, the default is 0 to avoid unintentional regularization
+static constexpr double defaultDiagRegularizeParameter = 0;
 
 // Note: move-capture, auto-return and auto-lambda-arguments require C++14
 /**