vtkSTLWriter and vtkSTLReader fails on ASCII files depending on locale

vtkSTLWriter uses fprintf to write out floating-point numbers. This is wrong for many reasons, the most serious is that it is locale-specific and so on a computer with European locale will create numbers with decimal comma instead of decimal point. All floating-point number writing in VTK should use doubleconversion library (see vtkXMLWriter.cxx).

Similarly, vtkSTLReader fails to read ASCII STL files on certain computers. See for example this error report: https://discourse.vtk.org/t/reading-ascii-stl-files-with-qvtk/3622/3?u=lassoan