Add dictionary parameter to the next_masker func

This does not change the default behavior, but now the function has
access to the distortion parameters over which the experiment loops.
This allow to, for example, pick a different masker according to a
'masker_type' key.
Use object attributes for column names

All the data frame column names are now defined as object attributes,
instead of being hard coded. They are not exposed in the `__init__`
function.
Use error level for log items
Add optional ax parameter to plot_results

It's mostly so that we can plot multiple results on the same figure.
Add comments about selecting plotting variable
Log changes to y scale when intelligibility
Remove get_srt function

It's redundant with srt_from_df, has a confusion name, and contains too
many hard-coded values.
Merge branch 'feature/experiment' into develop
Pred_to_pc converts predictions for specific model

The transformation from prediction to intelligibility is now model
specific, rather than it being applied to the whole dataframe. It's
possible to indicate if the transformation should be applied to a given
"Model" name, or a particulate "model" name and model "output" value.
Add possibility to force loading mono audio

Added an option to speech.Material to load audio files as mono, even if
they are stereo. This is to prevent errors in experiment where the
signal is "double" mono and there's a dimension mismatch.
Move pick_section in its own function

It makes the `pick_section` function available elsewhere, as well as
testable.
Fix definition of Hilbert transform

It seems like the code had suffered from a bad merge and a large
portion of the code was missing. I just pasted back the definition from
Scipy.
Add function to read wave files as floats

It is a thing wrapper around scipy.io.wavfile.read