hotfix: snafu from renaming ml_backends to processing_services

[mihow]

During the merging of #532 #684, something happened in which "ml_backends" was only partially renames to "processing_services" and we lost the precious "app.py" file. This prevented the ml_container from building and the tests from passing.

Some notes on naming:

• "Processing Services" is how we will call the servers (or endpoints) that host an API for processing images from Antenna. These can now be added by the user and hosted themselves. They run one or more processing pipelines. The services may be synchronous or asynchronous, they may manage their own queue, they may be a cluster or a single server.
• "ML Backend" is how we have been referring to the particular service for detecting and classifying moths with CNN models, like the one we have now.
• So the "processing_services" folder will have a few of these examples. The docker-compose.yml will likely have multiple processing services running soon. The current one is called "ml_backend", this could change to "moth_identifiers" or something more specific.

Feel free to add your thoughts, sorry for the bad merge!!!

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