Velox spectrum images: map Sample.elements as RosettaSciIO and HyperSpy do #45

RosettaSciIO maps Sample.elements for the EDS spectrum image of a Velox file (its own test: Test SI 16x16 215 kx.emd gives ["C", "O", "Ca", "Cu"]). VeloxSpectrumSignals maps it for single spectra since #44 (VeloxSampleElements, VeloxFileGroups); VeloxSpectrumImageSignals does not, and does not parse the four file-level groups on that path.

Plan: reuse VeloxFileGroups.read and VeloxSampleElements.convert in the spectrum-image path, refuse where RosettaSciIO or HyperSpy raises, extend tools/velox-elements/pins.py with spectrum-image records (the reference venv has sparse). Exact, as the single-spectrum pins.

Found while doing #10 item 2.

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