Add weighting to event branch of DiffractionFocussing

[peterfpeterson]

This issue was originally TRAC 6883

This has not been noticed by SNAP that if you use MaskBins on event data, that the number of spectra contributing to a d-range is not accounted for in DiffractionFocussing.

[peterfpeterson]

@NickDraper (2013-04-29T09:49:59):
Moved to r2.6 at the end of r2.5

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@NickDraper (2013-07-26T13:55:01):
Moved to backlog at the code freeze for R2.6

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@NickDraper (2014-02-14T11:04:59):
Bulk move to assigned at the introduction of the triage step

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@NickDraper (2014-12-08T10:25:31):
Moved to the backlog at the code freeze of R3.3

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@NickDraper (2015-04-27T08:10:34):
Moved to R3.5 at the R3.4 code freeze

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