Add constructor for MultiLevelTimeSeries class that uses initializer_list and add...

Add constructor for MultiLevelTimeSeries class that uses initializer_list and add stat methods that uses duration as a parameter.

Summary: Introduce a constructor that takes in an initializer list of 'TimeType' objects for the durations. New methods for sum/avg/rate/count/countrate are created with a 'TimeType' parameter - this makes it possible to call these methods using the duration that specifies the level. Previously, these methods could only be called using the index of the level in the levels_ vector. These methods first do a linear scan through levels_ using the method getLevelsByDuration to find the corresponding index, and then perform the function they specify on the level.

Differential Revision: D3414343

fbshipit-source-id: 8e1fcc16fd013d0b8b855a1eebbeff417e945b07