prep_hyp3: skip relative_orbit, start/stopUTC for isce2_burst products

[yunjunz]

Description of proposed changes

update prep_hyp3.py to calculate relative_obit, start/stopUTC for gamma_scene hyp3 products ONLY, as they are not available for for isce2_burst hyp3 products yet. Related to #1198.

• more comments
• light refactoring
• skip the relative_orbit, start/stopUTC calculation for isce2_burst

Reminders

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• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.