estimate_mixture does not exists anymore

[pradal]

In stat_tool, there are several tests that use the estimate_mixture function:

• test_data_transform.py : TestExtractData : test_histo_extract_data : line 279
• test_data_transform.py : TestExtractHistogram : test_mixture : line 338
• test_estimate.py line 58, line 73
• test_output.py line 34

AttributeError: '_DiscreteDistributionData' object has no attribute 'estimate_mixture'

Replace estimate_mixture by its equivalent