* track the prev guess in `geo2rdr`, break after convergence
* create doppler Poly once, avoid prints
Orbit.geo2rdr creates a poly, which prints "Polynomial Order: 0 - by - 0" each time
@falkamelung noted a race condition in #106 due to non-atomic directory
checking followed by os.makedirs. This pattern exists in many places
in the codebase so I went ahead and replaced the rest of them.
The new exist_ok usage should be terser and more idiomatic.
ChenZenghui
Scott Staniewicz
Paul A Rosen
Ryan Burns
stoormgeo
Eric Gurrola