Discuss and implement saving auxiliary join data

[gmanatole]

So far, joined data is only an attribute of Auxiliary class.
The dataframe should be saved as a csv in datarmor

[gmanatole]

@cazaudo For coding simplicity it can be saved in the acoustic data timestamps.csv
No more timestamps.csv in processed exists.
I propose that in data/audio/{len}_{sr} we store the processed auxiliary data (ie joined in existing timestamps.csv) and we leave raw aux data in data/auxiliary