[essreduce]: Allow single transformation timepoint

[nvaytet]

When we provide a transformation filter that slices a single value

wf = loki.LokiWorkflow()
wf[Filename[SampleRun]] = 'loki_051657_00000135.hdf'
wf[TransformationTimeFilter[sx.NXdetector, RunType]] = lambda da: da['time', 0].data

we ran into the error:

        if isinstance(total_transform, sc.DataArray):
            time_dependent = [t for t in chain if isinstance(t, sc.DataArray)]
>           times = [da.coords['time'][0] for da in time_dependent]
E           scipp._scipp.core.DimensionError: Slicing a scalar object is not possible.

We fix this by returning the .data if the result of the apply_transformation is a DataArray containing just a scalar value.

[jl-wynen]

To clarify, we got the error only when

wf[TransformationTimeFilter[sx.NXdetector, RunType]] = lambda da: da['time', 0]

i.e., without .data.