add tmp or cache for converted files

[skewballfox]

this is a bit different from my original plan for tmp. when I was testing changes the one operation that consistently took the longest amount of time was converting the wavs I had to mp3. We could make it to where we store the results in a tmp directory, which is wiped between reboots, in order to avoid converting it each time the project is run. if someone is testing or making multiple models using the same extra training data, then successive runs will be much faster, without cluttering their filesystem with multiple copies of data that have to be manually deleted.