STYLE: xoutcell should not flush its m_InternalBuffer
STYLE: xoutcell should not flush its m_InternalBuffer

There is no need to flush `xoutcell::m_InternalBuffer`, which is an `std::ostringstream`. As explained by Cubbi (Sergey Zubkov), http://www.cplusplus.com/forum/general/84962 November 15, 2012:

> flushing has no meaning for stringstream, as there is no external device that it sends data to

Also removed an unnecessary `using namespace` directive and an unnecessary `std::string::c_str()` call.

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix