feat: save start and end date of files

[tsutterley]

refactor: output rasters using structured netCDF4 function from geoid-toolkit
fix: include additional attributes to output files for CF compliance
refactor: spherical harmonic errors using euler's formula and np.einsum

[Copilot]

Pull request overview

This PR updates GRACE/GRACE-FO time metadata handling and output-file metadata, while also refactoring several spherical-harmonic-to-spatial computations and modernizing test formatting.

Changes:

• Add start/end date parsing as datetime/numpy.datetime64 metadata and propagate temporal coverage metadata into output products.
• Refactor raster/grid output writing (notably NetCDF4) to use a structured writer and align dimensions/attributes for improved CF-style compliance.
• Refactor some spherical-harmonic error/spatial summations using np.einsum and Euler-formula-based expressions; update/clean tests accordingly.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/test_units.py	Formatting/style updates for unit-factor tests.
test/test_time.py	Formatting/style updates for time conversion tests.
test/test_sea_level.py	Formatting/style updates for sea-level solver tests.
test/test_point_masses.py	Formatting/style updates for point-mass tests.
test/test_masks.py	Formatting/style updates for mask I/O tests.
test/test_love_numbers.py	Formatting/style updates for love-number reader tests.
test/test_legendre.py	Formatting/style updates for Legendre polynomial tests.
test/test_harmonics.py	Formatting/style updates for harmonic conversion/object tests.
test/test_gia.py	Improve robustness of GIA download fixture (xfail on network errors) + formatting.
test/test_download_and_read.py	Improve robustness of download tests (handle additional connection/login failures) + formatting.
test/conftest.py	Style/quoting updates for pytest CLI options/fixtures.
gravity_toolkit/time.py	Add parse() helper and return datetime objects from file parsers; add astype option to calendar_days; internal datetime import cleanup.
gravity_toolkit/scripts/scale_grace_maps.py	Add richer output attributes (incl. temporal/geospatial coverage) and refactor harmonic-error calculations with einsum/Euler approach.
gravity_toolkit/scripts/monte_carlo_degree_one.py	Comment clarifications aligned with new einsum-based summation approach.
gravity_toolkit/scripts/grace_spatial_maps.py	Add temporal/geospatial coverage attributes and minor refactors around output naming.
gravity_toolkit/scripts/grace_spatial_error.py	Add temporal/geospatial coverage attributes; refactor error-map computation and output object naming.
gravity_toolkit/scripts/grace_raster_grids.py	Switch to structured NetCDF4 output via geoid_toolkit; reorder output dimensions; add CF-style attributes.
gravity_toolkit/scripts/calc_degree_one.py	Comment clarifications aligned with new einsum-based summation approach.
gravity_toolkit/read_GRACE_harmonics.py	Use Python datetime for start/end date calculations; rename output dict variable for clarity; improve MMAX default handling.
gravity_toolkit/read_gfc_harmonics.py	Use Python datetime for start/end date calculations; rename output dictionary to gfc for consistency.
gravity_toolkit/grace_input_months.py	Store per-file start/end dates alongside monthly harmonics for later propagation into outputs.
gravity_toolkit/grace_date.py	Store start/end dates as numpy.datetime64 arrays when indexing GRACE files by month.
doc/source/notebooks/GRACE-Spatial-Error.ipynb	Update notebook computations to match new einsum/Euler-based approach and related API usage.

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