Multi-threading issues

This issue contains the problems that are not core to for multi-threading but still needs to be dealt with for it to be a complete feature.

1. • Aside from displaying tracking info, the parts of the code running in background threads cannot update the UI. The foremost example of this issue is with selecting key for DataTypeModel.
2. • Because the program does not freeze, the user can exit it at any time, sometimes forcefully. (not a problem anymore after #41 (closed) is solved)
3. • Waveform plot still has a lot of problems. If everything goes well, it works decently enough. If something goes wrong, however, the software fails with no chance of recovery.
4. • Some data sets cause data loading to fail with error messages that indicate a problem with threading. (Can't replicate)
5. • The program does not exit gracefully if zooming fails. (Folded into #66 (closed))
6. • Need to let user chooses the network if there are multiple in handling_data.read_soh_mseed. (Change in design make this a nonissue)
7. • Waveform processing threads do not exit gracefully when user quits the program.
8. • There should be update in waveform plot seeing as it can take a while to finish.