Return zero for non-existing well/completion

To be consistent with the general summary behaviour and more input
tolerant, 0.0 is returned when some phase, completion or well is
requested that isn't provided by the simulator.

Solves the issue discussed in https://github.com/OPM/opm-output/pull/122
and extends the test input deck to trigger this behaviour.

tests/summary_deck.DATA

@@ -530,3 +530,16 @@ TSTEP
 -- up historical rates and volumes. These volumes however don't change, i.e.
 -- every time step has the same set of values
 10 10 /
+
+-- Register a fourth well with completions later. This ensure we handle when
+-- wells are registered or activated later in a simulation
+WELSPECS
+     'W_4'        'G_3'   1    1  3.33       'OIL'  7* /
+/
+
+COMPDAT
+    W_4 1 1 3 3 /
+/
+
+TSTEP
+10 10 /