vtkm::io::reader::BOVDataSet fix 'constructor delegates to itself'

Fixes #517
7 jobs for deprecated_bov_reader_still_constructible in 0 seconds
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed cdash-commit

passed ghostflow-branch-check-master

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

00:10:03

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

00:18:51

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

00:21:51

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

00:25:25