Handle 'weight' for each MDEvent

[mantid-builder]

Original Reporter: @jzikovsky

The properly normalized signal for a box or a bin in MDHisto is:

Sum( event signal * event 'weight' ) / sum ( weights )

Where the weight of each 'event' is proportional to its solid angle * its width in energy transfer. For MDLeanEvent, weight would be 1.0 always. For MDEvent, it should either be cached or calculated on the fly (from the run number and detector ID).

• Each MDEvent carries run index and detector id which is enough to find the weight, but do we want to cache it ?
• Also , BinMD and the other integration methods would need to calculate the sum of the weight as they sum the signal.
• See also http://trac.mantidproject.org/mantid/ticket/4733 where BinMD needs to track it

[mantid-builder]

This issue was originally trac ticket 4734