Make Aquifer Data a First Class RestartValue

This commit promotes the 'aquifer' data member of 'RestartValue' to
first class status.  In particular, this means that users must
provide aquifer data, albeit possibly empty, when constructing the
RestartValue object for input or output.

This is in preparation of using more dynamic aquifer quantities from
the simulator layer as part of the restart I/O.

tests/test_restartwellinfo.cpp

@@ -229,7 +229,7 @@ BOOST_AUTO_TEST_CASE(EclipseWriteRestartWellInfo) {
                                      timestep,
                                      false,
                                      schedule.seconds(timestep),
-                                     Opm::RestartValue(solution, wells, group_nwrk));
+                                     Opm::RestartValue(solution, wells, group_nwrk, {}));
     }
 
     for (int i=1; i <=4; i++) {