Rewrite NexusFileIO and NexusClasses using NeXus::File api

[peterfpeterson]

This issue was originally TRAC 6625

The end state of http://trac.mantidproject.org/mantid/ticket/6518 was that there is more work to be done, but it is not required. This ticket is for continuing that work at a future time.

[peterfpeterson]

@NickDraper (2013-07-26T13:59:11):
Moved to the Backlog after the code freeze for R2.6

---

@NickDraper (2014-02-14T11:07:58):
bulk move to assigned at the into of the triage step

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you feel this is incorrect please comment to keep it alive, with a reason why.

To prevent closure, e.g. for long-term planning issues, add the "Never Stale" label.

[stale]

This issue has been closed automatically. If this still affects you please re-open this issue with a comment so we can look into resolving it.