Warning about slow loading of non-wav formats
**Is your feature request related to a problem? Please describe.**

Recently I've tried to understand why is my gpu usage spiking from 0 to 100% and why does training seem to halt every num_workers batches. What I found is that my dataset consisted of .opus and .mp3 files, and that they load 2-30 times slower than those same files when converted to .wav.

**Describe the solution you'd like**

A warning message that informs the user that non-wav files might become a bottleneck for training.

It seems wrong to show this message in segment.py cause it would be displayed for each of those files and pollute the logs.
Maybe the ManifestProcessor can keep a list of found formats and in the end of processing compare it to soundfile.available_formats()?
