Factor out compute/plotting/saving in `align_timestamps` functions

**Is your feature request related to a problem? Please describe.**
Currently, the `align_timestamps` does both the computation of the aligned timestamps, the plotting, and the saving.
This makes the code hard to reuse without a copy paste.

**Describe the solution you'd like**
Have the `align_*` functions only return aligned timestamps, and deal with additional functionalities elsewhere.

I can work on a PR for this :)