API add post-processing functions for activations

[tomMoral]

A particularly tricky part to look at the results in neuroscience in alphcsc is how to process the resulting activations. In particular, we could add:

• a helper to shift the activations' times to match the peak of an atom. Or maybe think of better alignment (start of non-zero atom?)
• a helper to compute epoched activations, that takes into account the event offset and output an Epoch object?

[tomMoral]

@CedricAllain maybe you are interested in tackling this issue?