Request : change public API methods taking std:string

Following the discussion at https://discourse.vtk.org/t/crash-in-tolowercase/5247/6, public API methods that take references to std::string as arguments can crash if VTK is not built with the same toolset as the one used to build an application linking to VTK. Here, it was between two gcc (and libstdc++) versions. I also encountered the same kind of issue on windows for some other projects, where a software and a library were not built using the same Visual Studio compilers, and crashed too. Those arguments should be replaced by char* buffers, or a vtk type used to handle strings, and not use a template type that might not have the same definition for both llib and application compiler The problematic method here was vtkNamedColors::setColor(). I don't know how widespread std::string arguments are, but i guess it is used in other places