ENH: make treatment of stream allocators more uniform (issue #532)
- use allocator class to wrap the stream pointers instead of passing them into ISstream, OSstream and using a dynamic cast to delete then. This is especially important if we will have a bidirectional stream (can't delete twice!). STYLE: - file stream constructors with std::string (C++11) - for rewind, explicit about in|out direction. This is not currently important, but avoids surprises with any future bidirectional access. - combined string streams in StringStream.H header. Similar to <sstream> include that has both input and output string streams.