FieldWriter: include missing headers

src/Structure/FieldWriter.cpp

 #include "FieldWriter.h"
 
-#include <fstream>
 #include <iomanip>
+#include <fstream>
 
 #include <boost/filesystem.hpp>
 #include <boost/format.hpp>
 
+#include "Algorithms/PBunchDefs.h"
+#include "Utilities/Util.h"
+#include "AbstractObjects/OpalData.h"
+
 template<typename FieldType>
 void FieldWriter::dumpField(FieldType& field, std::string name,
                             std::string unit, long long step,

src/Structure/FieldWriter.h

@@ -19,6 +19,8 @@
 #ifndef OPAL_FIELD_WRITER_H
 #define OPAL_FIELD_WRITER_H
 
+#include <string>
+
 class FieldWriter
 {
 public: