Change interface of `scale_curves_to_overlap`

[jokasimr]

Currently there is an optional argument return_scaling_factors that determines if the return value is a list of the scaled curves or if it is a list of the scaling factors to apply.

There was a discussion #90 (comment) about changing the interface.
The conclusion was that the simplest would be to remove the return_scaling_factors argument and instead always return both the scaled curves and the scaling factors.