Selecting the pixel type just means that lodepng casts it to that type
for it then to be cast to the type VTK-m uses. Since VTK-m always uses
the same type, there is little value for changing the intermediate cast.
Updated the image reader classes to better follow VTK-m naming
convention (i.e. use `ImageReaderPNG` instead of `PNGReader`).
The structure of the image reader class now also better matches
the interface for the VTK reader.
Also put the implementation of the image readers into the `vtkm_io`
library so that it only has to be compiled once.
