Adding MattesMutualInformation metric

Change-Id: I1b3e5728e48e470daffbffd4e466e24c1dfc6ca2
Adding MattesMutualInformation example and test

Change-Id: I9ee38d696b7ad093e67e3142ac959483357ec997
Merge topic 'SIMPLEITK-521_AddToImageRegistrationMethod' into next

b299417e Adding MattesMutualInformation example and test
de68e6ff Adding MattesMutualInformation metric