Variable-b. encoding: Allow several (equivalent) iterations per step

This means that a single step can be marked by /data/snapshot to
represent iterations 0,10,20,30 at the same time.
The underlying data is the same, but the API will treat it as 4 times a
different iteration with equivalent content.

src/ReadIterations.cpp

@@ -119,12 +119,12 @@ SeriesIterator::SeriesIterator( Series series )
         }
         }
 
-        if( !setCurrentIteration() )
+        if( status == AdvanceStatus::OVER )
         {
             *this = end();
             return;
         }
-        if( status == AdvanceStatus::OVER )
+        if( !setCurrentIteration() )
         {
             *this = end();
             return;

src/Series.cpp

@@ -1150,17 +1150,17 @@ Series::readGorVBased( bool do_init )
             return auxiliary::Option< std::deque< uint64_t > >();
         }
     }
-    case IterationEncoding::variableBased:
-    {
-        uint64_t index = 0;
+    case IterationEncoding::variableBased: {
+        std::deque< uint64_t > res = { 0 };
         if( currentSteps.has_value() && !currentSteps.get().empty() )
         {
-            // variable-based layout can only read one iteration at a time
-            // @todo warning or exception if the size is any other than 1?
-            index = currentSteps.get().at( 0 );
+            res = { currentSteps.get().begin(), currentSteps.get().end() };
+        }
+        for( auto it : res )
+        {
+            readSingleIteration( it, "", false );
         }
-        readSingleIteration( index, "", false );
-        return std::deque< uint64_t >{ index };
+        return res;
     }
     }
     throw std::runtime_error( "Unreachable!" );

test/SerialIOTest.cpp

@@ -4471,7 +4471,8 @@ void iterate_nonstreaming_series(
             auto E_x = iteration.meshes[ "E" ][ "x" ];
             E_x.resetDataset(
                 openPMD::Dataset( openPMD::Datatype::INT, { 2, extent } ) );
-            std::vector< int > data( extent, i );
+            int value = variableBasedLayout ? 0 : i;
+            std::vector< int > data( extent, value );
             E_x.storeChunk( data, { 0, 0 }, { 1, extent } );
             bool taskSupportedByBackend = true;
             DynamicMemoryView< int > memoryView;
@@ -4560,10 +4561,11 @@ void iterate_nonstreaming_series(
             iteration.close();
         }
 
+        int value = variableBasedLayout ? 0 : iteration.iterationIndex;
         for( size_t i = 0; i < extent; ++i )
         {
-            REQUIRE( chunk.get()[ i ] == int(iteration.iterationIndex) );
-            REQUIRE( chunk2.get()[ i ] == int(i) );
+            REQUIRE( chunk.get()[ i ] == value );
+            REQUIRE( chunk2.get()[ i ] == int( i ) );
         }
         last_iteration_index = iteration.iterationIndex;
     }