Seeding silence periods

[yger]

This PR adds the seeding property of the NoiseGeneratorRecording to the remove_silence function. To be more precise, I slightly extended the possibility of the NoiseGeneratorRecording to handle multiple noise_levels, such that internally, the remove_silence can make use of it to fill the portions of the signals where noise should be injected. By doing so, the noise is properly seeded and we are avoiding code duplication.

[yger]

Lots of file are changed, but this is only to extend the syntax of the NoiseGeneratorRecording, with noise_levelS instead of noise_level. This makes more sense to me I think