Fix for crash in multithreaded apps

[langford]

We get crashes in the FFNN functions from memory errors from this dispatching on a concurrent queue.

There is a simple fix here:

https://github.com/langford/SwiftOCR/commit/890eeae498f7dea177efbf1ca5c890ffd46393cc

That repo however is on a fork that already has swift 5 included.