Compose propagator breaks, if output length of not-last propagator is 1 but next takes any number

In the case that a propagator returns only one individual and the next propagator assumes its input to be a list of individuals, the next propagator will iterate over the dict features of the only propagator it got.

Thus, there will occur errors as soon as the propagator tries to access any member of the input individual.

Please add a workaround for this behaviour by either changing the propagators' interface so that all have to return lists of individuals or adding a check, that puts individuals that were returned without surrounding list back into one before plugging them into the next propagator.

This issue might be related to #61.

Start by locating the compose propagator entry point and the propagator interface; no file or test is named in the issue. Reproduce a composition where a non-final propagator returns one individual and the next expects a list, then verify that the next propagator receives a one-element list and no longer iterates over feature keys.

Written by the indexing model from the issue text.