Conform DataSet classes to coding practices better

The most common changes were making class members uppercase and spelled out, adding "this->" whenever a class member is used, and declare functions and members with export macros. Also fixed some uses of int (instead of vtkm::Id or something similar) and a bit of indentation. I also sprinkled some const goodness over the code.

It should be noted that I had about a week delay between first making these changes and checking them in. In the mean time Sujin also made some similar changes, so there might be some repetative changes.