Resolve Group/RetryProcedure overridden extension declaration [Swift 4]
Resolve Group/RetryProcedure overridden extension declaration [Swift 4]

`Retry.swift:134:24: Declarations from extensions cannot be overridden yet` (The Swift 4 compiler catches a case that the Swift 3 compiler was happy to allow.)

Resolved by moving the declaration of `child(_:didAttemptRecoveryFromErrors:)` in Group.swift from an extension of GroupProcedure to the main `class GroupProcedure`.
ProcedureKit/ProcedureKit,danthorpe/Operations,danthorpe/Operations,danthorpe/Operations,ProcedureKit/ProcedureKit,ProcedureKit/ProcedureKit