Move data loading to a separate thread

Create mechanism to load data in a separate thread.

Make plotting happens after data is loaded.

Add documentation to note that loadData and DataTypeModel.create_data_object are deprecated.

Add ability for DataLoader to deal with errors.

Note: there are still multiple things that need to be implemented for this feature to be complete. However, in the interest of keeping merge requests small, we only have the core parts of the feature in this merge request.