Add pytest test suite and GitHub Actions CI

[yumorishita]

Summary

LiCSBAS2 had no automated tests. This PR adds a pytest-based test suite and a GitHub Actions CI workflow. The whole suite needs no network and no real data, and runs in ~30 s (~5 s without the smoke marker).

• Unit tests for all LiCSBAS_lib modules (tools, inv, loop, io, plot): coordinate conversions, SBAS design matrices, NSBAS inversion (recovering a forward-modeled velocity field, incl. an n_core=2 vs n_core=1 consistency check), loop closure, raw-binary/GAMMA-par/baselines I/O, GeoTIFF round-trips, and plot smoke tests.
• End-to-end smoke tests (tests/test_bin_smoke.py, marker smoke): steps 11–16 and LiCSBAS_cum2vel.py run as subprocesses on a synthetic 10x10-px, 5-epoch, 7-ifg GEOCml dataset (tests/synth.py) with exactly loop-consistent phases from a known velocity field. The inverted velocity is checked against the truth within 0.1 mm/yr.
• Known latent bugs are pinned with xfail(strict=True), to be fixed in separate PRs:
  • inv_lib.censored_lstsq / censored_lstsq2 call the nonexistent np.linalg.leastsq in their 1-D/single-column branch
  • censored_lstsq2 accesses B.shape[1] before the B.ndim check (IndexError for 1-D input)
  • io_lib.make_geotiff raises UnboundLocalError for dtypes other than float32/uint8
• CI: micromamba with a CI-only conda-forge env file (.github/workflows/environment-ci.yml; LiCSBAS.yml is untouched), triggered on pushes and PRs to master, with environment caching.

Notes:

• The synthetic dataset adds tiny deterministic noise (sigma = 0.0003 rad, seed 42) because step 12's reference selection treats exactly-zero loop-phase RMS as nodata — a situation that cannot occur with real data.
• No packaging changes; LiCSBAS_lib is put on the import path by pytest.ini/conftest.py, so tests run without sourcing bashrc_LiCSBAS.sh.

Test plan

• python -m pytest in the licsbas conda env: 85 passed, 4 xfailed in ~30 s
• Same result with PYTHONPATH/MPLBACKEND unset (self-contained config)
• CI job on this PR solves the conda-forge env and passes

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