[FIX] Fix wrong assert in heatmap

[ales-erjavec]

Issue

Heatmap widget raises an error if a dataset with exactly 1000 instances in on input and 'Clustering (opt. ordering) is selected for Row clustering.

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 313, in _
    self.set_row_clustering(cb.itemData(idx, ClusteringRole))
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 528, in set_row_clustering
    self.__update_row_clustering()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 1089, in __update_row_clustering
    self.update_heatmaps()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 726, in update_heatmaps
    parts = self.construct_heatmaps(self.data, self.split_by_var, self.split_columns_var)
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 901, in construct_heatmaps
    ordered=self.row_clustering == Clustering.OrderedClustering
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 804, in cluster_rows
    assert len(matrix) < self.MaxOrderedClustering
AssertionError

This is due to an wrong condition in the assert

Description of changes

• Remove asserts

Includes

• Code changes
• Tests
• Documentation