[VTK][ascii] speed up output

[dglaeser]

When choosing ascii encoding, output is extremely slow in comparison to base64 or raw. The files are bigger, of course, but the difference is much larger than the file size difference.

One approach worth trying out could be to construct a string with the contents of a data array and then write the entire string at once - at the cost of a higher memory footprint. But ascii is not optimal for large files, anyway. And for small files the memory overhead doesn't matter that much...