Tool to trim the length of event NeXus files

[peterfpeterson]

This issue was originally TRAC 3181

In order to make system tests take less time, write a tool that takes an existing event NeXus file and copies it with less events.

[peterfpeterson]

@NickDraper (2011-06-27T08:20:29):
"New" tickets moved at the code freeze of iteration 29

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@NickDraper (2011-09-19T09:31:01):
Bulk move of tickets to iteration 31 at the iteration 30 code freeze

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@NickDraper (2012-01-09T09:43:40):
Moved to iteration 33 at iteration 32 code freeze

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@NickDraper (2012-04-30T14:14:57):
Moved at end of release 2.1

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@NickDraper (2012-08-10T12:45:05):
http://www.mantidproject.org/Release_Notes_2_2_Full_List

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@NickDraper (2012-10-28T11:39:21):
Moved to milestone 2.4

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@NickDraper (2013-01-28T09:23:27):
Moved at the code freeze for release 2.4

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@NickDraper (2013-04-29T09:50:04):
Moved to r2.6 at the end of r2.5

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@NickDraper (2013-07-26T13:55:02):
Moved to backlog at the code freeze for R2.6

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@NickDraper (2014-02-14T11:07:56):
bulk move to assigned at the into of the triage step

[NickDraper]

This has been done manually when required, and is better implemented in unit tests by not using files where possible