Workaround attribute issue with BP5

The Radiation plugin sets some attributes quite late. This is normally
supported by the openPMD-api, but can become a bit tricky, when the
attribute is a default attribute with a default value.
The Radiation plugin uncovers a bug in (depending on your point of view)
either openPMD-api or in ADIOS2 with this procedure in the BP5 engine.
Realistically the best fix for this would be to set defaults in the
openPMD-api as late as possible, and not as early as possible.
For now, defining these attributes as early as possible in the Radiation
plugin is a sensible workaround.

include/picongpu/plugins/radiation/Radiation.hpp

@@ -774,6 +774,13 @@ namespace picongpu
                     ::openPMD::Series& openPMDdataFile = series.value();
                     ::openPMD::Iteration openPMDdataFileIteration = openPMDdataFile.writeIterations()[currentStep];
 
+                    /* begin required openPMD global attributes */
+                    openPMDdataFileIteration.setDt<float_X>(DELTA_T);
+                    const float_X time = float_X(currentStep) * DELTA_T;
+                    openPMDdataFileIteration.setTime(time);
+                    openPMDdataFileIteration.setTimeUnitSI(UNIT_TIME);
+                    /* end required openPMD global attributes */
+
                     // begin: write amplitude data
                     ::openPMD::Mesh mesh_amp = openPMDdataFileIteration.meshes[dataLabels(-1)];
 
@@ -939,15 +946,6 @@ namespace picongpu
                     omega_mrc.storeChunk(detectorFrequencies, offset_omega, extent_omega);
                     // end: write frequencies
 
-                    /* begin openPMD attributes */
-                    /* begin required openPMD global attributes */
-
-                    openPMDdataFileIteration.setDt<float_X>(DELTA_T);
-                    const float_X time = float_X(currentStep) * DELTA_T;
-                    openPMDdataFileIteration.setTime(time);
-                    openPMDdataFileIteration.setTimeUnitSI(UNIT_TIME);
-                    /* end required openPMD global attributes */
-
                     openPMDdataFileIteration.close();
                     openPMDdataFile.flush();
                 }