Update CMake targets

Kenneth Morelandrequested to merge
kmorel/vtk-m-user-guide:cmake-options into master

VTK-m 2.0 changed the name of the exported targets to be in the vtkm namespace. Thus, library targets like vtkm_cont are now referenced like vtkm::cont.

Also did a quick pass through the build and install chapter to update features that were not properly documented.

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