organize application scripts on GitHub rather than disk

[mattersoflight]

@ziw-liu @edyoshikun I propose we organize the scripts that use VisCy (with other libraries) for specific applications in applications/infection_screens, applications/organelle_phenotyping, and applications/ultrack. I suggest this approach, because the scope of the repo is single-cell phenotyping. It is much easier to find code on git rather than disk, and encourages clean code organization.