Fix BinMD when used with a file-backed MD workspace

[martyngigg]

Fixes trac issue #11220

Tester

The following script will crash with the current version of master. After this fix the script should complete successfully. You can also run the MDWorkspaceTest system tests.

import os

from mantid.simpleapi import *

barefilename = "cncs.nxs"
try:
    os.remove(os.path.join(config["defaultsave.directory"], barefilename))
except OSError:
    pass

# Load then convert to Q in the lab frame
LoadEventNexus(Filename=r'CNCS_7860_event.nxs',OutputWorkspace='cncs_nxs')

ConvertToDiffractionMDWorkspace(InputWorkspace='cncs_nxs', OutputWorkspace='cncs_original', SplitInto=2)
alg = SaveMD(InputWorkspace='cncs_original', Filename=barefilename)

# Load into memory
LoadMD(Filename=barefilename,FileBackEnd='0',Memory='100',OutputWorkspace='cncs_mem')

# ======== File + mem, with write buffer ===========
LoadMD(Filename='cncs.nxs',FileBackEnd='1',Memory='100',OutputWorkspace='cncs_file')
PlusMD(LHSWorkspace="cncs_file", RHSWorkspace="cncs_mem", OutputWorkspace="cncs_file")
BinMD(InputWorkspace="cncs_file",AlignedDim0='Q_lab_x, -3, 3, 100',AlignedDim1='Q_lab_y, -3, 3, 100',AlignedDim2='Q_lab_z, -3, 3, 100',ForceOrthogonal='1',OutputWorkspace="test_binned")

DeleteWorkspace('cncs_file')

[martyngigg]

@FedeMPouzols Would you mind taking a look at this? It fixes the my ticket that you tested yesterday. A system test picked up a minor hole in the implementation.

[FedeMPouzols]

Nice, the example script is working well and the MDWTest passes now.
I'm running all system tests to make absolutely sure, and will merge this in.