Mantid crash caused by LET reduction scripts

[SilkeSchomann]

Original reporter: Mona Sarter (ISIS)

Describe the bug
Mantid crashes when running LET reduction scripts.

To Reproduce
I've got a copy of the reduction scripts. You will need PolCorr3 to run them.

[warunawickramasingha]

It has been identified that the crash was due to mantid running out of memory due to each run loading a number of workspaces into the memory.

@MialLewis
In terms of anything to be fixed, I propose mantidworkbench to have a feature to warn the user to free up the memory when the memory usage has reached a certain threshold(ex: 95%) that can be configured via the Mantid.user.properties file.