Fix BayesQuasi when sample workspace has numeric axis

[robertapplin]

Report to: Mona Sarter

Description of work

This PR fixes a bug in the BayesQuasi algorithm where an "index out of range" error would show when using a sample or vanadium with a NumericAxis rather than a SpectraAxis. ILL data has a NumericAxis, so this should make it possible to use this interface with this ILL data.

The problem was with the incorrect extraction of the Q values from the NumericAxis. It was previously calculating the centre value, but this was not necessary.

There is no associated issue.

To test:

1. Open Inelastic Bayes Fitting interface

2. Go to the Quasi tab

3. Load the following data as Sample and resolution. (You might need to unrestrict data by name in the Settings dialog)
   BCA299_Subtract.zip
   vanadium.zip

4. Click Run. If you get a pop-up, click Yes

5. The algorithm should run to completion. Previously a red error message would appear in the log saying "index out of range"

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[jhaigh0]

Test works with no error, code changes look good :)