2.0 DrillSettings object implements QuestionAnswerSettings

Settings of a drill.

Field	Type	Description
`direction` optional	String constant
`multipleChoiceSetSize` optional	String constant
`maxAnswerDuration` optional	ISO duration
`introductions` optional	String constant
`autoAdvancement` optional	String constant
`known` optional	2.0 QuestionAnswerStyle object
`unknowns` optional	2.0 QuestionAnswerStyle object