Add drivers for spectrometers

[barlowmj]

Goal

Add support for spectrometer-like instruments that return array-valued data (e.g., counts vs pixel/energy) to MeasureIt's sweep framework. This enables Sweep0D, Sweep1D, Sweep2D, etc. to follow ParameterWithSetpoints parameters alongside existing scalar parameters.

Background

QCoDeS already supports array-valued parameters via ParameterWithSetpoints (recommended) and ArrayParameter (legacy). The DataSaver.add_result() automatically unpacks array parameters with their setpoints. MeasureIt currently blocks these parameters because Arrays validator has is_numeric = False, and the plotter silently flattens arrays to a single element.

Implementation Plan

What changes

1. src/measureit/tools/util.py — Fix is_numeric_parameter() to accept Arrays validator and ParameterWithSetpoints/ArrayParameter types. Add is_array_parameter() helper.

2. src/measureit/_internal/plotter_thread.py — For array-valued parameters, plot np.sum(array) as the y-value (total signal intensity) with "(sum)" label. Full spectrum data is saved to the database via QCoDeS.

3. tests/conftest.py — Add MockSpectrometer class using ParameterWithSetpoints (Gaussian peak + configurable noise).

4. tests/unit/test_array_params.py — Comprehensive tests for array param support and backward compatibility.

What doesn't change

• base_sweep.py data path — safe_get() → add_result() already works because QCoDeS DataSaver auto-unpacks ParameterWithSetpoints
• _create_measurement() registration — QCoDeS register_parameter() auto-discovers internal setpoints via depends_on and merges with sweep setpoints
• heatmap_thread.py — Receives pre-reduced scalar data from plotter, no changes needed

Future work

• Live heatmap visualization for array params in Sweep1D (x=sweep, y=pixel, color=counts)
• 3D visualization for Sweep2D with array params
• Configurable reduction function (sum, max, mean, custom)
• Real hardware spectrometer drivers (Andor, Ocean Optics, etc.)