Fix bug when opening a filebased series as groupbased

In that case, we must not attempt to piece back together the filename by
filebased methods, but instead directly use the filename specified by
the user.

Author: franzpoeschel

include/openPMD/Series.hpp

@@ -86,6 +86,7 @@ class SeriesData : public AttributableData
 
 OPENPMD_private :
     auxiliary::Option< WriteIterations > m_writeIterations;
+    auxiliary::Option< std::string > m_overrideFilebasedFilename;
     std::string m_name;
     std::string m_filenamePrefix;
     std::string m_filenamePostfix;

src/Series.cpp

@@ -913,6 +913,12 @@ SeriesImpl::readGroupBased( bool do_init )
                 series.m_iterationEncoding = IterationEncoding::fileBased;
                 std::cerr << "Series constructor called with explicit iteration suggests loading a "
                           << "single file with groupBased iteration encoding. Loaded file is fileBased.\n";
+                /*
+                 * We'll want the openPMD API to continue series.m_name to open
+                 * the file instead of piecing the name together via
+                 * prefix-padding-postfix things.
+                 */
+                series.m_overrideFilebasedFilename = series.m_name;
             } else
                 throw std::runtime_error("Unknown iterationEncoding: " + encoding);
             setAttribute("iterationEncoding", encoding);
@@ -1065,6 +1071,10 @@ std::string
 SeriesImpl::iterationFilename( uint64_t i )
 {
     auto & series = get();
+    if( series.m_overrideFilebasedFilename.has_value() )
+    {
+        return series.m_overrideFilebasedFilename.get();
+    }
     std::stringstream iteration( "" );
     iteration << std::setw( series.m_filenamePadding )
               << std::setfill( '0' ) << i;