Use named parameters when constructing a Message

[LourensVeen]

When constructing a message, we have Message(1.0, None, data). That None is the next timestamp t_next, which we don't use here. It would be better to write Message(1.0, data=data), then we don't have to explain t_next at all.

This requires a change in MUSCLE3, see multiscale/muscle3#146. After that, we can update the lesson.

Thanks to @sverhoeven for the idea!