downloading orbit

[LiangJYu]

This PR puts the thinnest of wrappers on @scottstanie sentineleof module to download EOF modules from ESA or ASF.

The following script demonstrates usage:

from s1reader.s1_orbit import download_orbit_file

safe_no_ext = 'S1A_IW_SLC__1SDV_20200511T135117_20200511T135144_032518_03C421_7768.'

# test both zip file and SAFE directory
for ext in ['zip', 'SAFE']:
    downloaded = download_orbit_file(f'{safe_no_ext}{ext}')
    print(downloaded)

[LiangJYu]

Closing in lieu of a solution with fewer dependencies.