[Tingliangstu]

Saved partial SED components can now be combined for plotting and fitting, for example plot_partial_SED = Sr+Ti x+z or plot_partial_SED = all x. This addresses the element-group and whole-system directional workflows discussed in #63 and #81 through postprocessing of existing output_partial = 1 files.

Changes

• Add + selections for elements and Cartesian directions; omitted directions mean x+y+z. Preserve the existing Sr and Sr x behavior and figure names.
• Add save_partial_SED = 1 by default to export only the selected sum beside the figures. Set it to 0 to skip export. Saved sums retain the full frequency grid and linear intensities; original components are unchanged.
• Read components one at a time, exclude derived sums from all, and report invalid selections, missing components, mismatched shapes, and inconsistent scales.
• Update the manual and add seven small tests covering sums, exports, compatibility, error handling, actual plot data/rendering, and fitting input.

SED computation, FFTs, normalization, and CPU/GPU kernels are unchanged. Only code, documentation, and tests are included; no trajectory data, calculation results, or generated images are uploaded. This PR is independent of the q-path changes in #101.

Validation

• Windows: python -m unittest tests.test_partial_sed tests.test_pipeline_io tests.test_cli -q — 46 tests passed.
• WSL pysed environment: python -m unittest tests.test_partial_sed -q — 7 tests passed.
• Strict Sphinx HTML build (-W --keep-going) and git diff --check passed.
• Compatibility with Support explicit disconnected q paths in pySED 3.0 #101: Git merged both branch tips without conflicts; all 14 partial-SED and q-path tests passed on that merged tree in a temporary directory.
• Tests ran with BLAS/OpenMP thread limits set to 1 and small temporary data.
• GitHub's push CI passed Linux Python 3.10/3.12/3.13, documentation, and wheel checks. Windows Python 3.12 reports the existing text-progress byte-count failure (264 != 256 in tests/io/test_text.py:161, two batch-size subtests); the same failure is present on the unchanged 3.0 base. The feature-branch job passed the other 122 tests. This PR does not change that Windows test or its trajectory reader.