Replace std::stringstream with vtkValueFromString in vtkVariant::ToX conversion functions, when the variant hold a string.
This will increase performances of all code that depends on these functions such as vtkStringToNumeric, vtkDelimitedTextReader and more.
The behavior is the same except that [-]infinity is no longer supported ([-]inf is still supported). Let me know if I should add an edge case to maintain the exact same behavior or if it was not used in practice.