text: Title: There is an error when I use this model on image classification.
  Body: import  numpy as np
  import os
  import glob
  from skimage import io,transform
  import matplotlib.pyplot as plt
  from copy import deepcopy
  from sklearn.decomposition import PCA
  from sklearn.datasets import load_iris
  from sklearn.ensemble import RandomForestClassifier
  from modAL.models import ActiveLearner,Committee
  
  #数据集地址
  path = 'E:/data/datasets/flower_photos/'
  #模型保存地址
  model_path = 'E:/data/model/model.ckpt'
  
  #将所有图片重置为100*100
  w = 100
  h = 100
  c = 3
  #读取图片
  def read_img(path):
      cate = [path+x for x in os.listdir(path) if os.path.isdir(path+x)]
      imgs = []
      labels = []
      for idx,folder in enumerate(cate):
          for im in glob.glob(folder+'/*.jpg'):
              print('reading the images:%s'%(im))
              img = io.imread(im)
              img = transform.resize(img,(w,h))
              imgs.append(img)
              labels.append(idx)
      return np.asarray(imgs,np.float32),np.asarray(labels,np.int32)
  data,label = read_img(path)
  shape = np.shape(data)
  
  #产生池
  X_pool = deepcopy(data)
  y_pool = deepcopy(label)
  
  #初始化委员会
  n_members = 3
  learner_list = list()
  
  for member_idx in range(n_members):
      #初始化训练集
      n_initial = 100
      train_idx = np.random.choice(range(X_pool.shape[0]),size=n_initial,replace=False)
      X_train = X_pool[train_idx]
      y_train = y_pool[train_idx]
      #去除训练集之后的数据集
      X_pool = np.delete(X_pool,train_idx,axis=0)
      y_pool = np.delete(y_pool,train_idx)
  
      #初始化学习器
      learner = ActiveLearner(estimator=RandomForestClassifier(),X_training=X_train,y_training=y_train)
      learner_list.append(learner)
  
  Traceback (most recent call last):
    File "E:/query_by_committee/query_by_committee.py", line 56, in <module>
      learner = ActiveLearner(estimator=RandomForestClassifier(),X_training=X_train,y_training=y_train)
    File "D:\Anaconda3\lib\site-packages\modAL\models.py", line 104, in __init__
      self.X_training = check_array(X_training)
    File "D:\Anaconda3\lib\site-packages\sklearn\utils\validation.py", line 451, in check_array
      % (array.ndim, estimator_name))
  ValueError: Found array with dim 4. Estimator expected <= 2.
  
  I am a novice, so the problem may be stupid, please excuse me.
  
  
  