Add UTClusterCreator_Monitor and adapt to UTDigit (@xuyuan)

• UTClusterCreator_Monitor is a new temporary algorithm, constructing UTClusters from UTDigits, only will be used for the monitoring.
• Use of UTDigit->clone() is removed (following LHCb!2631 (merged))
• The unused UTVeloDigitsMerge algorithm is removed (following LHCb!2631 (merged))

The changes here were originally part of !504 (merged). See there for discussions and review.

Goes together with LHCb!2631 (merged), Boole!317 (merged), Rec!2240 (merged), Moore!695 (merged)