Add MaximumMeanDiscrepancy metric

[kzkadc]

Description: added maximum mean discrepancy (MMD) metric, which measures the discrepancy of two distributions. https://www.onurtunali.com/ml/2019/03/08/maximum-mean-discrepancy-in-machine-learning.html

Check list:

• [x] New tests are added (if a new feature is added)
• [x] New doc strings: description and/or example code are in RST format
• [x] Documentation is updated (if required)