Refactor 6_Remote_Sensing and 7_Pathfinding notebooks (#1001)

Remote Sensing: self-contained with synthetic spectral bands from
terrain elevation. Covers true color, NDVI, SAVI, ARVI, EVI, NBR,
and NDMI.

Pathfinding: redesigned with a 20x20 open grid so connectivity
differences are immediately visible. 8-conn gives a clean diagonal,
4-conn gives a staircase, friction-weighted curves around expensive
cells.

Author: brendancol