Add Means of Retrieving Node Names from Extended Network

This commit adds a new member function

    ExtNetwork::node_names() const

which collects the names (std::string) of all nodes in a network.
The immediate use case is creating summary config nodes for network
level quantities specified without explicit lists of nodes, e.g.,

    GPR
    /

Add a simple unit test for demonstration.

tests/parser/NetworkTests.cpp

@@ -17,10 +17,12 @@
   along with OPM.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#define BOOST_TEST_MODULE ScheduleTests
+#define BOOST_TEST_MODULE NetworkTests
 
 #include <boost/test/unit_test.hpp>
 
+#include <algorithm>
+
 #include <opm/parser/eclipse/EclipseState/Schedule/Network/ExtNetwork.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/Network/Node.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/Network/Branch.hpp>
@@ -32,6 +34,7 @@
 
 using namespace Opm;
 
+namespace {
 Schedule make_schedule(const std::string& schedule_string) {
     Parser parser;
     auto python = std::make_shared<Python>();
@@ -43,7 +46,7 @@ Schedule make_schedule(const std::string& schedule_string) {
     Schedule schedule(deck, grid , fp, runspec, python);
     return schedule;
 }
-
+}
 
 
 BOOST_AUTO_TEST_CASE(CreateNetwork) {
@@ -266,3 +269,50 @@ BRANPROP
     }
 }
 
+BOOST_AUTO_TEST_CASE(NodeNames) {
+    const auto sched = make_schedule(R"(
+SCHEDULE
+
+GRUPTREE
+ 'PROD'    'FIELD' /
+
+ 'M5S'    'PLAT-A'  /
+ 'M5N'    'PLAT-A'  /
+
+ 'C1'     'M5N'  /
+ 'F1'     'M5N'  /
+ 'B1'     'M5S'  /
+ 'G1'     'M5S'  /
+/
+
+BRANPROP
+--  Downtree  Uptree   #VFP    ALQ
+    B1         PLAT-A    9999      1*      /
+    C1         PLAT-A    9999      1*      /
+/
+
+NODEPROP
+--  Node_name Pr    autoChock?      addGasLift?     Group_name
+     PLAT-A 21.0   NO     NO    1*  /
+     B1    1*  YES      NO    1*  /
+     C1    1*  YES     NO     'GROUP' /
+/
+
+TSTEP
+  10 /
+
+BRANPROP
+--  Downtree  Uptree   #VFP    ALQ
+    C1         PLAT-A    0 1*      /
+/
+)");
+
+    const auto expect = std::vector<std::string> {
+        "B1", "C1", "PLAT-A"
+    };
+
+    auto nodes = sched.network(0).node_names();
+    std::sort(nodes.begin(), nodes.end());
+
+    BOOST_CHECK_EQUAL_COLLECTIONS(nodes.begin(), nodes.end(), expect.begin(), expect.end());
+}