NavMap ObstacleFilter: mark the cell under each point

[fmrico]

Fixes #114

The NavMap ObstacleFilter grouped the sensor points in fixed 0.3 m columns and marked the NavCel under the column centre, up to ~0.21 m away from the obstacle. Each (already downsampled, downsample_resolution) point is now checked where it is: the NavCel under it, and its height above that NavCel. The hard-coded 0.3 m column is gone.

Validation

In the Summit warehouse, the laser no longer marks a line of obstacles ~0.25 m in front of the shelf:

• before: x ≈ 2.53;
• after: x ≈ 3.07–3.13, inside the shelf, whose face is at 2.78.

The robot reaches the goal as before, with no extra late RT cycles.

Tests

• New: a thin post marks only the cells it stands on, not the neighbouring one where a column centre would fall.
• Adjusted to the new semantics: four existing tests placed their points on a cell border and checked the column-centre cell. They now put the points at the centre of the cell they check; what each test verifies (range, minimum height, growth with distance) is the same.
• 2D-laser bin test: with the default 0.3 m downsampling one point per voxel survives, so it now checks that the bin's cells are marked.