perspective_remap.py: new module that inverts the terrain-reference warp
in TERRAIN-DISP-MERGED frames using ELEV_GND depth (64×80 tiles, tilted
plane, no NaN). Produces natural-parallax perspective images per scene,
valid only in the overlap with the reference camera's ground footprint.

elphel_to_colmap_perspective.py: new CLI script (parallel to the original
elphel_to_colmap.py, which is kept unchanged) that adds --elev_gnd and
applies the remap before writing COLMAP output.

Tested on 185-frame NC dataset: valid-pixel min=0.10, mean=0.54, max=1.00.
SciPy added to Dependencies in README.
Co-authored-by: Claude <claude@elphel.com>