Add code to internalize GPMAINT keyword in Group objects

tests/parser/GroupTests.cpp

@@ -537,3 +537,62 @@ BOOST_AUTO_TEST_CASE(GCONINJE_GCONPROD) {
         BOOST_CHECK( g2.injectionGroupControlAvailable(Phase::GAS));
     }
 }
+
+BOOST_AUTO_TEST_CASE(GPMAINT) {
+    const auto input = R"(
+SCHEDULE
+
+GRUPTREE
+ 'PROD'    'FIELD' /
+
+ 'M5S'    'PLAT-A'  /
+ 'M5N'    'PLAT-A'  /
+
+ 'C1'     'M5N'  /
+ 'F1'     'M5N'  /
+ 'B1'     'M5S'  /
+ 'G1'     'M5S'  /
+ /
+
+GPMAINT
+  'PROD'  'WINJ'   2  1*  100  0.25  1.0 /
+  'C1'    'GINJ'   0  1*  100  0.25  1.0 /
+/
+
+TSTEP
+   10 /
+
+GPMAINT
+  'PROD'  'NONE' /
+/
+
+)";
+    Opm::UnitSystem unitSystem = UnitSystem( UnitSystem::UnitType::UNIT_TYPE_METRIC );
+    double siFactorG = unitSystem.parse("GasSurfaceVolume/Time").getSIScaling();
+    const auto sched = create_schedule(input);
+    {
+        const auto& prod_group = sched.getGroup("PROD", 0);
+        const auto& plat_group = sched.getGroup("PLAT-A", 0);
+        const auto& c1_group = sched.getGroup("C1", 0);
+
+        const auto& gpm_prod = prod_group.gpmaint();
+        BOOST_CHECK( gpm_prod );
+        BOOST_CHECK(gpm_prod->flow_target() == GPMaint::FlowTarget::RESV_WINJ);
+
+        auto [name, number] = *gpm_prod->region();
+        BOOST_CHECK_EQUAL(number, 2);
+        BOOST_CHECK_EQUAL(name, "FIPNUM");
+
+        const auto& gpm_c1 = c1_group.gpmaint();
+        BOOST_CHECK(!gpm_c1->region());
+
+        const auto& plat_prod = plat_group.gpmaint();
+        BOOST_CHECK( !plat_prod );
+    }
+    {
+        const auto& prod_group = sched.getGroup("PROD", 1);
+        const auto& gpm_prod = prod_group.gpmaint();
+        BOOST_CHECK( !gpm_prod );
+    }
+}
+