Improve adduct/isotope generation

[joewandy]

Summary

This PR improves simulated isotope envelopes, adduct generation, and deisotoping/deadducting support.

The main scientific change is replacing the previous carbon-only isotope approximation with a multi-element isotope envelope approximation based on natural isotope abundances. The PR also adds configurable adduct priors/profiles and concentration controls, removes adducts we cannot currently simulate correctly, and adds OpenMS / ms-deisotope helpers plus tests to validate generated patterns.

Scientific fixes and behavior changes

Isotope generation

Isotope generation now approximates the full-formula isotope envelope by convolving per-element natural isotope distributions (NATURAL_ISOTOPES).

• Build each element distribution relative to its monoisotope.
• Raise each element distribution to the element count via repeated convolution.
• Merge/prune intermediate states with mass_precision, min_prob, and max_states to keep runtime bounded.
• Keep peaks until total_proportion is reached.
• max_peaks now defaults to unlimited; if explicitly set too low to reach total_proportion, a warning is emitted.
• Always preserve the zero-shift monoisotopic peak as isotopes[0], because downstream code assumes that invariant.

Implementation lives in vimms/IsotopeDistribution.py, with vimms/Chemicals.py::Isotopes kept as the public wrapper.

Adduct generation

Adduct metadata now has one shared definition table in vimms/Common.py, from which names, priors, and mass terms are derived.

• Adduct proportions are sampled from a Dirichlet distribution with configurable priors/profiles.
• Default positive/negative adduct sets are derived from the shared table.
• Custom adduct names are validated, including ionisation-mode compatibility.
• The mislabeled potassium adduct M+2K+H was replaced with M+2K-H.
• Negative-mode adduct coverage now includes M+Cl, M+FA-H, and M+Ac-H; M+Cl uses the chloride ion mass.
• Unsupported dimer/multimer adducts (2M+H, 2M+NH4) were removed because the old mass-term approach applies isotope envelopes incorrectly for multimer species. It is better not to simulate them than to produce scientifically wrong isotope/adduct peaks.

Deisotoping and deadducting

vimms/Deisotoping.py is added with utilities for validating and processing generated isotope/adduct patterns.

• A lightweight deisotoper is added and tested against multi-element isotope envelopes.
• OpenMS and ms-deisotope helper paths are added and covered by tests.
• OpenMS deadducting support is implemented with focused helper functions for feature-map setup, adduct parsing, probability normalization, and neutral-mass reconstruction.
• pyopenms and ms-deisotope are added as required dependencies for this PR's functionality and tests.

Validation

Added tests/test_deisotoping.py to cover:

• multi-element isotope envelope generation,
• monoisotope preservation under filtering,
• warning behavior when max_peaks truncates the requested envelope,
• removal/rejection of unsupported dimer adducts,
• potassium replacement and chloride adduct definitions,
• adduct metadata consistency from the shared table,
• polarity-aware adduct validation,
• OpenMS and ms-deisotope helper behavior,
• end-to-end deadducting examples.

Local validation run in the vimms conda environment:

• pytest tests/test_deisotoping.py -q -> 30 passed
• pytest tests/test_chemical_generation.py tests/test_controllers_TopN.py -q -> 31 passed
• pytest -q -> 144 passed, 2 existing warnings
• Smoke simulation: generated HMDB-derived chemicals, isotope/adduct envelopes, and non-empty Top-N MS1/MS2 scans.

CI is currently green on the PR.

Limitations

• True dimer/multimer adduct isotope envelopes are not supported in this PR. They were removed rather than approximated incorrectly.
• The adduct prior defaults are heuristic and configurable; they are not claimed to be empirically calibrated for all instruments or sample classes.
• Unknown chemicals remain single-peak representations. This is intentional because ROI-derived unknowns may already represent a specific isotope/adduct peak or noise, and expanding them could invent correlated signal.