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/FileInterface/RifCsvUserData.cpp

@@ -166,3 +166,11 @@ void RifCsvUserData::buildTimeStepsAndMappings()
         }
     }
 }
+
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+size_t RifCsvUserData::keywordCount() const
+{
+    return m_mapFromAddressToResultIndex.size();
+}