Allow RAiDER to fall back to ASF for Sentinel-1 orbits

[jhkennedy]

Right now, our orbit credential checking function:
https://github.com/dbekaert/RAiDER/blob/dev/tools/RAiDER/s1_orbits.py#L19-L46

• only checks ESA CDSE credentials
• throws an error if ESA credentials aren't found

But sentineleof is capable of calling back to ASF via NASA EDL credentials to download orbits if it cannot find them from ESA.

So we should probably instead:

• check for ESA CDSE credentials
• check for NASA EDL credentials
• warn if one exists but not the other
• throw if neither exists