Refactor into installable pyncmap package

[SminYu]

Motivation

• Reorganize the repository into a proper Python package so NCL-style *.rgb/*.bound colormaps can be consumed directly from Matplotlib-coded projects.
• Provide a clearer, more robust API for loading colormaps, handling common input formats, reversing maps, and surfacing actionable errors.

Description

• Introduce pyncmap package with core implementation in src/pyncmap/core.py exposing ColorMapMaker, get_colormap, and list_available_colormaps, and define ColormapFileError for missing resources.
• Implement robust RGB parsing, automatic 0–1 normalization (detects 0–255 or 0–1 inputs), _r-suffix reverse support, and BoundaryNorm creation via .norm and .boundaries properties.
• Package example colormap data under src/pyncmap/cmap_data/ and add project metadata and packaging configuration in pyproject.toml with package-data to include the *.rgb/*.bound files.
• Add convenience backward-compatible shim ncl_colormap.py, tests in tests/test_core.py, updated README.md, LICENSE, and a .gitignore for developer hygiene.

Testing

• Ran python -m compileall src ncl_colormap.py tests which completed successfully.
• Ran PYTHONPATH=src pytest -q which failed to collect/run because numpy is not installed in this environment (tests require numpy and matplotlib).
• Attempted python -m pip install -e . which failed in this environment due to inability to fetch build dependencies (setuptools>=68) from the package index because of network/proxy restrictions.

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Codex Task