Field data feature and some other small changes needed for using Fides in VTK/ParaView

The last MR needed for the Fides reader in VTK. The largest change is adding a FieldData class that is used to store variables that is not associated with points or cells and so shouldn't be stored in the vtkm::cont::DataSet.

The other smaller changes include:

• Handling ADIOS StepStatus returned from BeginStep(). Previously we weren't actually checking this, now we do as well as returning a status from DataSetReader::PrepareNextStep(). Since we check all DataSources in that call, we now loop on a DataSource if its status is NotReady, so we don't get into a potentially buggy situation where one DataSource may be ready but not another. So PrepareNextStep() only actually returns a status of OK or EndOfStream. It only returns EndOfStream when all DataSources have reached EndOfStream, otherwise it returns OK. In a situation with multiple sources, users shouldn't need to worry about when a single DataSource reaches EndOfStream, because ADIS handles this internally (e.g., having the variables making up a mesh marked as static and only reading on the first step).
• The StorageXGC classes and related ArrayHandles were updated so they are not read only. This allows for deriving new XGC Fields like turbulence and adding it to the DataSet.
• An example python script for visualizing XGC using Paraview Simple API is now included under examples/paraview.