Change shape of dataframe of `extract_dataframe` in morph_stats (#874)

neuron features in morph_stats (#874)

The features in the 'neuron' entry of the config dict will not be run on each neurite_type, but on the whole neuron only once. This avoid duplicating features if they are the same on all neurite types, such as the ones in neuronfunc.py

The returned dataframe is now of the form:
```  
       | property  | axon                  | all                   | neuron
       | name      | feature_1 | feature_2 | feature_1 | feature_2 | feature_3
   -------------------------------------------------------------------------------------
    0  | simple    | 0.1       | 0.2       | 0.15      | 0.25      | 1000
```