Filtering in Mantid reduction backend based on meta data

[SimonHeybrock]

Mantid supports this, e.g., via the two algorithms GenerateEventsFilter and FilterEvents. There are a few questions regarding interaction with the stream, however:

• Can meta data be late? Jon says (2015-11-11): Aggregator will work with one pulse delay and send meta data together with corresponding pulse. So we can work under that assumption for now (but this may change).
• Consider a slowly changing variable, e.g., temperature. Will its value be transmitted for each pulse, or not? If not the Mantid reduction backend must be capable of inferring its value from previously received frames. See #17.

[SimonHeybrock]

This is bascially identical to #20, closing it for now.