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

@@ -115,12 +115,12 @@ SeriesIterator::SeriesIterator(Series series) : m_series(std::move(series))
         }
         }
 
-        if (!setCurrentIteration())
+        if (status == AdvanceStatus::OVER)
         {
             *this = end();
             return;
         }
-        if (status == AdvanceStatus::OVER)
+        if (!setCurrentIteration())
         {
             *this = end();
             return;

src/Series.cpp

@@ -1240,15 +1240,16 @@ std::optional<std::deque<uint64_t> > Series::readGorVBased(bool do_init)
         }
     }
     case IterationEncoding::variableBased: {
-        uint64_t index = 0;
+        std::deque<uint64_t> res = {0};
         if (currentSteps.has_value() && !currentSteps.value().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.value().at(0);
+            res = {currentSteps.value().begin(), currentSteps.value().end()};
         }
-        readSingleIteration(index, "", false);
-        return std::deque<uint64_t>{index};
+        for (auto it : res)
+        {
+            readSingleIteration(it, "", false);
+        }
+        return res;
     }
     }
     throw std::runtime_error("Unreachable!");

test/SerialIOTest.cpp

@@ -5031,7 +5031,8 @@ 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;
@@ -5119,9 +5120,10 @@ 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(chunk.get()[i] == value);
             REQUIRE(chunk2.get()[i] == int(i));
         }
         last_iteration_index = iteration.iterationIndex;