Remove const qualifier so it can be built with FCC

[ikitayama]

As of today the HEAD can not be built with FCC on the frontend for K computer. Please review and pull the patch into master.

[heplesser]

@ikitayama Thank you for reporting this issue. The root of the problem is a inconsistency in the C++98 standard. Probably due to an oversight, basic_ofstream::is_open() is not a const method in the C++98 standard (§27.8.1.10), even though the underlying basic_filebuf::is_open() method is const (§27.8.1.3). In C++11 and later, is_open() is const also for basic_ofstream. It appears that the FCC compiler is the only one adhering strictly to the C++98 standard here.

[heplesser]

@ikitayama Thank you for reporting this issue. The root of the problem is a inconsistency in the C++98 standard. Probably due to an oversight, basic_ofstream::is_open() is not a const method in the C++98 standard (§27.8.1.10), even though the underlying basic_filebuf::is_open() method is const (§27.8.1.3). In C++11 and later, is_open() is const also for basic_ofstream. It appears that the FCC compiler is the only one adhering strictly to the C++98 standard here.

[heplesser]

@ikitayama Thank you for your pull request. May I kindly ask you to add a comment explaining the background for this change (see my inline comment).