Data populated ratio

[Gautzilla]

🐋 What's new?

This PR adds some stuff to the API to help the user filtering out data:

Property/Method name	Description	Done
BaseData.populated_duration	Return the total duration of non-empty items	✅
BaseData.populated_ratio	Return the ratio between the duration of non-empty items and the duration of the data	✅
SpectroData.populated_duration	Return the total duration of non-empty items if the SpectroData has associated files, otherwise the populated_duration of the linked AudioData	✅
BaseDataset.remove_data_on_populated_ratio(threshold=0.)	Remove all data that have a populated ratio lower than threshold	✅

🐬Note

This PR resove #295

[Gautzilla]

@mathieudpnt