Rename app.py examples to app-core.py

[gshotwell]

We have a standardized way of naming template files with app-core.py and app-express.py. This changes our api examples to follow that pattern. This has a few beenfits:

• Clear which file is which
• Easier to keep express and core examples in sync if they're in the same place
• Express and core apps can share submodules (e.g. plotting helpers)

[gadenbuie]

Thanks! Looks like we're getting errors that might be due to a recent update from black? They're definitely unrelated to the changes in this PR, so I'm going to merge.