replace hardwired parameters in unwrap_fringe.py

[yunjunz]

The current version of unwrap_fringe.py has the following three parameters hardwired, using typical values from Sentinel-1. They should be extracted from the metadata files instead, like the unwrap.py from ISCE/topsStack.

altitude = 800000.0
earthRadius = 6371000.0
wavelength = 0.056

Mark here for the to-do-list.