Fix measure_dr_pref

This pull request fixes measure_dr_pref, which previously resulted in an error. This was because _feature_list in measure_dr_pref should return a list of Features, however the used Duration was a subclass of Dimension rather than Feature.

This change then needs values to be a list rather than float.

Finally, an unrelated error occurred when using 
> Orientation(values=or_values, compute_fn=compute_orientation_from_direction)

Because compute_fn is used, the name of this object is used in line 347 of \_\_init\_\_.py:
> complete_settings = permuted_settings +\
>                            [(f.name, f.compute_fn(permuted_settings))
>                             for f in self.features_to_compute]

This results in "Orientation". However, the lower-case name is expected in _featureresponses.

I don't know whether it is best to put the lower() where it is currently or changing
> f.name

in

> f.name.lower()

Best, Tobias