Fix VTKDataSetWriter writing `char` types and NaN

1. Cast char types to int before writing
2. NaN values were being written as "nan" which caused read error in VTK
readers. Now we just write the large float value.
5 jobs for fix-dataset-writer in 0 seconds (queued for 1 second)
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:01:22

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:00:22

passed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:06:27

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:01:53