Signal compression

[romainbrette]

Currently, the Board can save recordings in compressed .npz format (lossless). Since we have continuous signals, I figured that storing temporal differences might be more efficient. I added a save_compressed() method which does that and gets 25% additional lossless reduction on a real example. Issues to address are:

• the signals dictionary is changed in place
• we need a load or decompress function
• we need to make sure there is no significant propagation of numerical errors

[romainbrette]

I think it might not be such a great idea, because of propagation of errors.