Fix issue with single realization when using resdata (#12737)

* Moves multi reader initialization
* Log error message if not able to open file
* Make keywordCount pure virtual

The base class `RifSummaryReaderInterface` is changed to be abstract and the `keywordCount` is declared as a pure virtual method, therefore the derived classes must implement the method.

This will ensure that all readers are responsible for reporting a keyword count.

* Ensures addresses are created if required

Calls `createAddressesIfRequired` on the summary reader to ensure addresses are created before accessing keyword count.
This prevents potential issues when the addresses haven't been initialized yet.

ApplicationLibCode/ProjectDataModel/Summary/RimEnsembleStatisticsCase.h

@@ -54,7 +54,8 @@ public:
     static RiaDefines::DateTimePeriod findBestResamplingPeriod( time_t minTimeStep, time_t maxTimeStep );
 
 private:
-    void clearData();
+    void   clearData();
+    size_t keywordCount() const override;
 
 private:
     std::vector<time_t> m_timeSteps;