Release v0.6.1 — patch release with mixed-dtype GeoTIFF fix

[profsergiocosta]

Context

PR # (test/io-viz-coverage) fixed a data-corruption bug in
dissmodel/io/raster.py::_write_geotiff: the GeoTIFF was created with
a single dtype taken from the first band (arrays[0].dtype). With
mixed-dtype band_specs (e.g. uso int32 + alt float32), float bands
were silently truncated to int. The fix promotes all bands to
np.result_type(*dtypes) when dtypes are mixed. A regression test
(tests/io/test_raster_io.py::test_mixed_dtype_bands_are_not_truncated)
covers it. Test coverage rose from 55% to 79% (319 → 441 tests).

A data-loss fix in I/O warrants an immediate patch release.

Tasks

• Bump version to 0.6.1 in pyproject.toml
• Add CHANGELOG entry under ## [0.6.1] (Fixed: mixed-dtype
  GeoTIFF truncation; Added: io/visualization test suites,
  coverage 55% → 79%)
• Note in CHANGELOG: GeoTIFFs with mixed-dtype bands saved with
  v0.6.0 may have truncated float bands — re-export recommended
• Document dtype promotion in save_geotiff docstring
  (GeoTIFF format limitation: single dtype per file)
• Tag v0.6.1 and push (Trusted Publishing handles PyPI)

Acceptance criteria

• pip install dissmodel==0.6.1 available on PyPI
• CHANGELOG renders correctly on GitHub
• Full suite green: 441 passed, coverage ≥ 79%