Switch Time of Flight units in Spectrum Viewer

[MikeSullivan7]

Issue

Closes #2097.

Description

In the Spectrum Viewer, the spectrum plot can be used in different Time of Flight related units, i.e. Neutron Energy, Neutron Wavelength via a right click menu. This menu is greyed out and the "Image Index" unit is selected with no viable sprectra.txt loaded.

Testing

make check

Acceptance Criteria

• Load data in MI with a spectra file (e.g. Brass dataset)
• Right click the spectrum plot and switch to the various units and check that the data displays correctly.
• Move the range window around to check that the "Image index range" label under the plot responds appropriately. Also check that the averaged image in the image view behaves as expected.
• Move the range window from its default position and change the selected unit, check that the range window resizes to the new unit range.
• With the appropriate data loaded, check the Normalise to open beam checkbox and see if the plots display correctly.
• Load any data without a spectra file and switch to that data via the Sample dropdown menu in the SV.
• Check that the spectrum plot displays with the image index selected as the unit in the right click menu,. The right click menu should also be greyed out to prevent bugs.

Documentation

Added release notes

[coveralls]

coverage: 73.956% (-0.3%) from 74.286%
when pulling 4eb97a0 on 2097_Tof_wavelength_plot
into b6087e8 on main.

[MikeSullivan7]

Comparing the results of MI to those published for the FE Powder:

Mantid Imaging

Published Data

Note: The position of the Bragg Peaks can move to higher or lower wavelength depending on the temperature of the sample. I dont know what temperature this example data was taken but the positions of the peaks look reasonable

[samtygier-stfc]

Looking good. A few small code things.

[samtygier-stfc]

Just a small thing:
If I load 2 datasets, one with and one without spectra values, open the spectrum viewer, select an image stack with spectra, change to wavelength, then change the image stack to the one without. The axis label still says "Wave length"

[samtygier-stfc]

Looks good. Thanks