Rename SRT parameter in utils.int2srt

The name `srt_at` makes more sense than just `srt` because it means "SRT
at X %".
Add function to calculate SRTs from predictions

It works if the SRTs calculation function applies to all groups (e.g.
Model, Output, Condition), which is usually the case. However, the
function does not work if the parameters are different for each
condition, such as for the STEC model (Wan et al. 2010 and 2014). In
that case, it would be possible to loop over the different parameters
values required for the conversation to SRT and to call this function
repeatedly.
Change the default output folder to `output`
Add param to set levels before or after distortion

It was a common problem that some experiments, especially binaural ones,
required to adjust target and noise levels at the source, rather than at
the ears. With this new parameter, it's not necessary to rewrite the
whole `preprocessing` function anymore, just to change the order of the
level adjustment.