Upon parsing, store each iteration's filename

If the padding is inconsistent, a later Iteration::open() needs the
original filename. Trying to compute the filename from the expansion
pattern will lead to wrong filenames.

include/openPMD/Iteration.hpp

@@ -249,6 +249,11 @@ class Iteration : public LegacyAttributable
             std::make_shared<auxiliary::Option<DeferredParseAccess> >(
                 auxiliary::Option<DeferredParseAccess>());
 
+    std::shared_ptr<auxiliary::Option<std::string> >
+        m_overrideFilebasedFilename =
+            std::make_shared<auxiliary::Option<std::string> >(
+                auxiliary::Option<std::string>());
+
     /**
      * @brief Begin an IO step on the IO file (or file-like object)
      *        containing this iteration. In case of group-based iteration

src/Iteration.cpp

@@ -360,6 +360,8 @@ void Iteration::readFileBased(
     auto &series = retrieveSeries();
 
     series.readOneIterationFileBased(filePath);
+    *m_overrideFilebasedFilename =
+        auxiliary::makeOption<std::string>(std::move(filePath));
 
     read_impl(groupPath);
 }

src/Series.cpp

@@ -35,6 +35,7 @@
 #include <iostream>
 #include <regex>
 #include <set>
+#include <sstream>
 #include <string>
 #include <tuple>
 
@@ -1142,14 +1143,34 @@ void SeriesInterface::readBase()
 
 std::string SeriesInterface::iterationFilename(uint64_t i)
 {
+    /*
+     * The filename might have been overridden at the Series level or at the
+     * Iteration level. See the struct members' documentation for the reasons.
+     */
     auto &series = get();
+    auto iteration = series.iterations.find(i);
     if (series.m_overrideFilebasedFilename.has_value())
     {
         return series.m_overrideFilebasedFilename.get();
     }
-    std::stringstream iteration("");
-    iteration << std::setw(series.m_filenamePadding) << std::setfill('0') << i;
-    return series.m_filenamePrefix + iteration.str() + series.m_filenamePostfix;
+    else if (
+        iteration != series.iterations.end() &&
+        iteration->second.m_overrideFilebasedFilename->has_value())
+    {
+        return iteration->second.m_overrideFilebasedFilename->get();
+    }
+    else
+    {
+        /*
+         * If no filename has been explicitly stored, we use the filename
+         * pattern to compute it.
+         */
+        std::stringstream iterationNr("");
+        iterationNr << std::setw(series.m_filenamePadding) << std::setfill('0')
+                    << i;
+        return series.m_filenamePrefix + iterationNr.str() +
+            series.m_filenamePostfix;
+    }
 }
 
 SeriesInterface::iterations_iterator