Investigate time slicing with ISIS SANS scaled background subtraction

[rbauststfc]

Under issue #35446 we added functionality to perform a scaled background subtraction via the ISIS SANS GUI (see https://docs.mantidproject.org/nightly/interfaces/isis_sans/Runs%20Tab.html#scaled-background-subtraction). There was some follow-on work to allow the background workspace to be specified using just the row output name, as a sort of "smart look-up" feature.

There are three realistic scenarios where a scaled background subtraction would be used:

1. Background not sliced and sample not sliced
2. Background not sliced and sample sliced
3. Background sliced and sample sliced

Scenario 1

This correctly supports output names as a "smart look-up" function and there is nothing further we need to look at for this scenario.

Scenario 2

This currently does not support output names as a “smart look-up” function, but it should. This would mean adding the following logic to the existing logic for looking up matching workspaces in the ADS:

1. Find workspace names that match the Background Workspace string + only the default suffixes (Bank, Mode, Wavelength range)

Scenario 3

For this scenario, we need to ask the wider group whether it is sufficient for people’s needs that only the first slice from the background workspace is currently subtracted from all samples (i.e. for sample slices = t0_t600, t600_t1200, t1200_t1800, then the subtraction will currently always use background t0_t600). It may be necessary (and more intuitive when using a “smart look-up”) for sample slices to instead have the corresponding background slice subtracted. However there would be complexities (scientifically at the very least) that mean any implementation of this is likely to be pretty significant.

If the decision is that we go for a change in the existing implementation of scenario 3, then we can confirm with the group that the “smart lookup” should perform the “slice matching” version of the subtraction in this case. If the decision is to stick with the current “first slice only” subtraction, then we can consider whether it may be better not to have a “smart lookup” for scenario 3 at all in order to avoid misunderstandings over what is being done.