Use FileBackedExperimentInfo class in LoadMD

[martyngigg]

Fixes trac issue #11111

Tester

The major work was completed in #307. This simply uses the new class in LoadMD when the FileBackend option is set to true. The best way to test is to see the time difference in loading a large MD file before and after the changes. A 24Gb test file can be found on the Vates file share at ISIS. Let me know if you want me to point you at it.

[martyngigg]

Jenkins, retest this please.

[FedeMPouzols]

All looks good, including system tests.

I did a quick non-rigorous comparison using the large 22413_22932_SQW.nxs (~24 GB) file. I'd say that that's more than enough to verify that 'FileBackEnd' does what it is supposed to do:

• With FileBackEnd enabled: 58.72 s to finish a lazy LoadMD. Goes down to 37.84 s once the file is buffered.
• Without: 1579.46 s for a traditional, full load. I was going to stop it as It gets awfully slow when it runs out of RAM on a 16GB machine and starts using swap space.