Allow flushes before defining position and positionOffset records

This will ensure openPMD specific requirements for those records at flush time as soon as those records are present -- rather than at creation time of the particle species object.
Currently missing is any kind of check that those records are present before closing the series.

Author: franzpoeschel

include/openPMD/ParticleSpecies.hpp

@@ -55,9 +55,6 @@ namespace traits
         template< typename T >
         void operator()(T & ret)
         {
-            /* enforce these two RecordComponents as required by the standard */
-            ret["position"].setUnitDimension({{UnitDimension::L, 1}});
-            ret["positionOffset"].setUnitDimension({{UnitDimension::L, 1}});
             ret.particlePatches.linkHierarchy(ret.m_writable);
 
             auto& np = ret.particlePatches["numParticles"];

src/ParticleSpecies.cpp

@@ -122,6 +122,17 @@ ParticleSpecies::flush(std::string const& path)
             for( auto& patch : particlePatches )
                 patch.second.flush(patch.first);
         }
+
+        iterator it = find("position");
+        if ( it != end() )
+        {
+            it->second.setUnitDimension({{UnitDimension::L, 1}});
+        }
+        it = find("positionOffset");
+        if ( it != end() )
+        {
+            it->second.setUnitDimension({{UnitDimension::L, 1}});
+        }
     }
 }
 } // openPMD