Performance: lazy-load material data on first access instead of at import

[Copilot]

All 5 default materials (H, D, T, C12, Be9) were loaded eagerly at import NeSST, adding ~500ms of I/O overhead even when only primary spectra were needed.

Changes

• constants.py: Replace mat_dict = {} with mat_dict = LazyMaterialDict() — a dict subclass that loads material data on first key access via __missing__. Uses a deferred import of initialise_material_data from core to avoid circular imports at module-load time.
• core.py: Remove the eager loading loop (for mat in default_mat_list: initialise_material_data(mat)).
• tests/test_core.py: Add tests asserting mat_dict is a LazyMaterialDict, loads and caches on first access, and raises KeyError for unknown labels.

Behaviour

import NeSST as nst

# mat_dict is empty at import — no material I/O performed
assert len(nst.mat_dict) == 0

# Primary spectra work immediately with no material data loaded
mean, _, _ = nst.DTprimspecmoments(5e3)

# Material data loaded on first access, cached thereafter
mat_d = nst.mat_dict["D"]       # triggers load
mat_d2 = nst.mat_dict["D"]      # returns cached object instantly
assert mat_d is mat_d2

All modules sharing mat_dict via from NeSST.constants import * (including time_of_flight) reference the same LazyMaterialDict instance, so lazy loading is consistent across the package.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Performance: default materials loaded at import</issue_title>
<issue_description>Currently, all the default material data are loaded when NeSST is imported. This is slow if you just want primary spectra. A performance improvement would be to only load the data once it is required, i.e. once a call is made to the material data class for that material</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Performance: default materials loaded at import #60

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