Fix rounding in vtkXMLReader(Writer)::UpdateProgressDiscrete to avoid FE_INEXACT

This PR fixes member function UpdateProgressDiscrete found in vtkXMLReader and vtkXMLWriter. The function currently uses truncation from float to int as part of rounding to nearest percent. However, as demonstrated by this example, such truncation causes an FE_INEXACT floating point exception to be raised on x86_64 platforms (under a wide variety of compilers and optimization levels). If VTK is used in a code that enables FPE trapping, any attempt to read or write a mesh may raise an FPE signal.

The proposed solution is to simply use std::round from <cmath> which performs correct rounding without raising the FPE.