Refactor SaveMD/LoadMD/MergeMD files to use the same code and increase unit test coverage for this part of the code

[abuts]

This issue was originally TRAC 8510

new MDNode interface allows almost not to use templates in these MD algorithms and share common parts of the code between all three algorithms.

The shared code should be unit tested.

[abuts]

@NickDraper (2014-02-14T11:07:54):
bulk move to assigned at the into of the triage step

---

@abuts (2014-05-02T15:28:24):
Mantid developers discussion concluded that the current behaviour is fine. Spectra limits are set from binning range and this is desirable behaviour for event workspace too.

---

@abuts (2014-05-02T15:29:50):
invalid comment to invalid ticket

[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.