Fix: prevent scientific notation in GVI outputs (#89) + Add DC demo dataset (#62)

[AaronMZhou]

Fixes #89.
Forces GVI to 6-decimal float at calculation and normalizes before merge/write to avoid scientific notation in CSV/GeoJSON/GPKG.

[AaronMZhou]

Thanks for reviewing!
This PR addresses #89 and #62.
#89: Formats GVI to 6 decimals at calculation and normalizes before merge/write to avoid scientific notation in CSV/GeoJSON/GPKG.
#62: Adds a small Washington, DC demo roads dataset at data/demo_dc/DC_roads_sample.geojson and README section (“Washington, DC Demo Dataset”).
Note: one earlier commit message mistakenly references #63; this PR is for #62.