calc rel_orbit from abs_orbit

[yunjunz]

Grab the relative and absolute orbits info for LT1-A/B from all the available SLCs, and estimate the equation, in the form of:

rel_orbit_num = (abs_orbit_num - orbit_num_offset) % num_cycles + 1

Reference:

• https://github.com/isce-framework/s1-reader/blob/bb585c77ee538324212085f4737f3d2d534ac998/src/s1reader/s1_burst_slc.py#L1115
• [New Feature]: Support for S1C isce-framework/s1-reader#141