text: Title: [BUG]  AttributeError: 'GroupedPredictor' object has no attribute 'predict_proba'
  Body: I am not sure if it's me that I am not using correctly the API.
  
  When doing a classification problem and using a GroupedPredictor to estimate probabilities I can't get the probabilities
  
  ```python
  from sklego.meta import GroupedPredictor
  import numpy as np
  from sklearn.linear_model import LogisticRegression
  from sklego.datasets import load_chicken
  
  df = load_chicken(as_frame=True)
  
  # Create a binary target
  X = df.drop(columns='weight')
  y = np.where(df.weight>df.weight.mean(),1,0)
  
  mod = GroupedPredictor(LogisticRegression(), groups=["diet"])
  
  mod.fit(X,y)
  
  mod.predict_proba(X)
  ```
  
  
  `
  Traceback (most recent call last):
    File "bug.py", line 25, in <module>
      mod.predict_proba(X)
  AttributeError: 'GroupedPredictor' object has no attribute 'predict_proba'
  `
  