Refactor vtk-m symbol visibility macros to be more consistent.

There really isn't any reason to use the CMake generated export macro header as it can easily be written in a cross platform way inside VTK-m.

Secondly instead of labeling each method that needs to have visibility we can label entire classes.

Lastly the CMake generated export macro functionality would break if we every tried to generate export macros from the cont or exec directory, as those defines already exist to label device/host code.