Allow timelines to be defined on [0, m] where m is < n_nodes-1 #8
Description
As per title.
Currently, it's not possible to define, for example, a force regularization phase and add it to a standalone timeline. This is because the phase manager tries to add the phase to the last node and everything explodes. A "hack" is to simply allow shorter timelines so that new phases can enter or exit the horizon (if pushed back) at a previous node