GDOS save_ascii outputting data in single line fix

[mducle]

Description of work:

As noted in PR#987, a GDOS cut is "tranposed" such that the data is align along the second dimension. The code to extract a set of x, y, e from the MDHisto workspace explicitly assumes that it is along the first dimension and so fails. This PR fixes this.

To test:

Run this script:

import mslice.cli as mc

ws = mc.Load(Filename='MAR21335_Ei60meV.nxs', OutputWorkspace='MAR21335_Ei60meV')
cut_ws = mc.Cut(ws, CutAxis="DeltaE,0.0,55.0,0.25", IntegrationAxis="|Q|,4.0,10.0,0.0", NormToOne=False, IntensityCorrection='gdos', SampleTemperature=300.0)
mc.save_ascii(cut_ws, 'cut_gdos.txt')

cut_ws = mc.Cut(ws, CutAxis="DeltaE,0.0,55.0,0.25", IntegrationAxis="|Q|,4.0,10.0,0.0", NormToOne=False, IntensityCorrection=False, SampleTemperature=None)
mc.save_ascii(cut_ws, 'cut_sqe.txt')

and check that both output text files have three data columns. The datafile is included in Mantid test files.

Fixes #1004.

[SilkeSchomann]

This fix works well and code looks good 👍🏻