Changed the default behavior of msr2data for writing output-file headers.

The default (if no option is given) is now that only for newly generated or empty files the respective header will be written.
If data is appended to an existing file, it is assumed that the header also is present already!
In this case only the new data blocks are appended directly after to existing ones.
[Most probably this behavior is broken if used in a native Windows environment, however, this is not the only problem there...]

The previous option "noheader" is preserved.
It suppresses the output of the header in any case.
If new data are appended to an existing output file this is done at the end of the file---just as before!

A new option "header" has been introduced.
If this is given, the output of the header is forced---no matter if a file (probably with header) existed before or not.
Also in this case all new data (and the header) are appended at the end of the output file if it existed already.

In case both options are given, the default behavior is activated.

src/include/PMsr2Data.h

@@ -73,7 +73,7 @@ class PMsr2Data
     bool PrepareNewInputFile(unsigned int, bool) const; // template
     bool PrepareGlobalInputFile(unsigned int, const string&, unsigned int) const; // generate msr-input file for a global fit
 
-    int WriteOutput(const string&, bool, bool, bool global = false, unsigned int counter = 0) const;
+    int WriteOutput(const string&, bool, unsigned int, bool global = false, unsigned int counter = 0) const;
 
   private:
     bool PrepareNewSortedInputFile(unsigned int) const; // template