Loading progress bar

[annehaley]

A collaborator suggested that our current indeterminate loading spinner can be limiting to a user who doesn't know how long a render should take. If possible, we should try to display more detailed progress information.

There are several steps involved in the full loading time, but the most time consuming is the persist call on the sliced_data_array in pvxarray. We will need to find a way to gauge the progress of this call.

We already know the expected size of the data, so if we can regularly measure the size of the sliced_data_array during the persist call, we can determine the percentage of loading completion.

[johnkit]

Would also be useful to inform the user of progress from (i) data download, (ii) building vtk objects, (iii) rendering