Automatically call `initialize` in `__init__` of `finam.Composition`

All the coupling scripts we have look like this:

composition = fm.Composition([comp_1, comp_2])
composition.initialize()

And it seems strange, that initialize needs to be called after the composition was created. Nothing other could happen between creation and initialization.

I think the content of initialize could just be moved to __init__ and we can drop this method.