Update oemof-solph >= 0.5.0

A recent update of oemof.solph (0.5.0) seems to imply a change of declaration of some function.

Transformer Source Sink

Are now used like this :

oemof_solph.Source => oemof_solph.components.Source

oemof_solph.Sink => oemof_solph.components.Sink

oemof_solph.Transformer => oemof_solph.components.Transformer

@l.gontier do you think it needs to be change in master ? I suppose these declaration doesn't work with oldest version of oemof_solph (< 0.5.0)..