reflectometry: New detector routine should allow multiple or no spectra set

As a LARMOR instrument scientists I would like the detector manager `NormalisedIntensityDetector` to be more versitile so that I can do more experiments.

### Acceptance Criteria

1. ~Allow multiple spectra definitions to be used for the monitor and detector spectra. The multiple spectra are to be summed.~ Completed, see https://github.com/ISISComputingGroup/IBEX/issues/5472
1. Allow monitor spectra to be set an empty set at which point the calulcation is no longer an average but just a sum

Start by locating the NormalisedIntensityDetector detector manager and inspect how monitor spectra are currently handled. Implement support for an empty monitor-spectrum set so the calculation uses a sum rather than an average, then verify the existing multiple-spectrum work remains unaffected. The issue names no file or test, so identify the relevant test coverage while tracing this entry point.

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