Query all IDs of (FIP) regions in State

Enable support for querying what fluid-in-point regions are defined in
the deck. For now the only interesting regions are the ones specified by
the FIPNUM keyword in the REGIONS section.

The implementation is somewhat inefficient (n log n over the number of
cells in the grid), but since this is likely to only be called when
default-expanding SUMMARY section keywords this isn't too bad, since the
implementation is so dead simple.

opm/parser/eclipse/EclipseState/tests/EclipseStateTests.cpp

@@ -580,3 +580,31 @@ BOOST_AUTO_TEST_CASE(TestIOConfigCreationWithSolutionRPTSOL) {
         BOOST_CHECK_EQUAL(true, ioConfig->getWriteRestartFile(0));
     }
 }
+
+BOOST_AUTO_TEST_CASE(getRegions) {
+    const char* input =
+            "START             -- 0 \n"
+            "10 MAI 2007 / \n"
+            "RUNSPEC\n"
+            "\n"
+            "DIMENS\n"
+            " 2 2 1 /\n"
+            "GRID\n"
+            "DXV \n 2*400 /\n"
+            "DYV \n 2*400 /\n"
+            "DZV \n 1*400 /\n"
+            "REGIONS\n"
+            "FIPNUM\n"
+            "1 1 2 3 /\n";
+
+    const auto deck = Parser().parseString(input, ParseContext());
+    EclipseState es( deck, ParseContext() );
+
+    std::vector< int > ref = { 1, 2, 3 };
+    const auto regions = es.getRegions( "FIPNUM" );
+
+    BOOST_CHECK_EQUAL_COLLECTIONS( ref.begin(), ref.end(),
+                                   regions.begin(), regions.end() );
+
+    BOOST_CHECK( es.getRegions( "EQLNUM" ).empty() );
+}