Default extra_order in error estimation to 1

Our default quadrature rules integrate polynomial degree 2p+1 exactly,
which is good enough for even a mass matrix assembly, but which *isn't*
good enough to take an L2 norm of the order-p+1 error term in a degree-p
approximate solution.

include/error_estimation/exact_error_estimator.h

@@ -155,15 +155,16 @@ class ExactErrorEstimator : public ErrorEstimator
    */
   void attach_reference_solution (EquationSystems * es_fine);
 
-
   /**
    * Increases or decreases the order of the quadrature rule used for numerical
-   * integration.
+   * integration.  The default \p extraorder is 1, because properly
+   * integrating L2 error requires integrating the squares of terms
+   * with order p+1, and 2p+2 is 1 higher than what we default to
+   * using for reasonable mass matrix integration.
    */
   void extra_quadrature_order (const int extraorder)
   { _extra_order = extraorder; }
 
-
   // Bring the base class functionality into the name lookup
   // procedure.  This allows for alternative calling formats
   // defined in the base class.  Thanks Wolfgang.

include/error_estimation/exact_solution.h

@@ -177,7 +177,10 @@ class ExactSolution
 
   /**
    * Increases or decreases the order of the quadrature rule used for numerical
-   * integration.
+   * integration.  The default \p extraorder is 1, because properly
+   * integrating L2 error requires integrating the squares of terms
+   * with order p+1, and 2p+2 is 1 higher than what we default to
+   * using for reasonable mass matrix integration.
    */
   void extra_quadrature_order (const int extraorder)
   { _extra_order = extraorder; }

src/error_estimation/exact_error_estimator.C

@@ -52,7 +52,7 @@ ExactErrorEstimator::ExactErrorEstimator() :
     _exact_deriv(nullptr),
     _exact_hessian(nullptr),
     _equation_systems_fine(nullptr),
-    _extra_order(0)
+    _extra_order(1)
 {
   error_norm = H1;
 }

src/error_estimation/exact_solution.C

@@ -45,7 +45,7 @@ namespace libMesh
 ExactSolution::ExactSolution(const EquationSystems & es) :
   _equation_systems(es),
   _equation_systems_fine(nullptr),
-  _extra_order(0)
+  _extra_order(1)
 {
   // Initialize the _errors data structure which holds all
   // the eventual values of the error.