Orbit from header

[hfattahi]

Checked for duplicates

Yes - I've already checked

Alternatives considered

Yes - and alternatives don't suffice

Related problems

Currently the reader only works if the external orbit file is provided.

Describe the feature request

Since the annotation file already contains the less precise orbit, I think the default behavior should be to extract the orbit from external orbit if the orbit file is provided. If the orbit file is not provided let's just extract the orbit from the header (annotation).

In the annotation file, the orbits are in <generalAnnotation/orbitList> . isce2 implementation can be reused.