Dump mesh stl binary

There is a bug when one tries to dump binary stl files. The dump class opens the file in textmode and not in binary mode.
I fixed this with an own openfile method in dump_mesh_stl.cpp which checks for writeBinarySTL_ flag.
if (filewriter) {
if (writeBinarySTL_) {
fp = fopen(filecurrent,"wb");
} else {
fp = fopen(filecurrent,"w");
}
everything else is like in the parent class.

On my case with 1570 triangles the dump binary stl is two times faster than ASCII dump !

Best,
Christian.