Support writing binary files to legacy VTK files

Kenneth Morelandrequested to merge
kmorel/vtk-m:write-binary into master

The legacy VTK file writer writes out in ASCII. This is helpful when a human is trying to read the file. However, if you have more than a trivial amount of data, the file can get impractically large. To get around this, VTKDataSetWriter now has a flag that allows you to write the data in binary format.

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