do not catch polymorphic type `Dune::RangeError` by value

This addresses the following compiler warning from GCC 8:

    warning: catching polymorphic type ‘class Dune::RangeError’
    by value [-Wcatch-value=]

dune/common/test/densematrixassignmenttest.cc

@@ -244,7 +244,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -255,7 +255,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -266,7 +266,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -281,7 +281,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -295,7 +295,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -309,7 +309,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -342,7 +342,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -352,7 +352,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }
@@ -362,7 +362,7 @@ bool run() {
       std::cout << "(line " << __LINE__ << ") Error: No exception thrown."
                 << std::endl;
       passed = false;
-    } catch (Dune::RangeError) {
+    } catch (const Dune::RangeError&) {
       std::cout << "(line " << __LINE__
                 << ") All good: Exception thrown as expected." << std::endl;
     }