Rename MixIn into Mixin
Due to an oversight, there is an inconsistency in the namign of mixin classes. See !436 (comment 134465) This MR fixes this inconsistency by renaming "MixIn" into "Mixin"
Due to an oversight, there is an inconsistency in the namign of mixin classes. See !436 (comment 134465) This MR fixes this inconsistency by renaming "MixIn" into "Mixin"