Fix unwrapping error in differential interferogram between A and B

[oberonia78]

Issue

Phase jumps were observed in the ionosphere layer due to unwrapping errors (reported by Marin).

Root Cause

• During the computation of the differential interferogram between frequency A and B, the phase difference is directly calculated between the two bands without mask.
• During the unwrapping step, the subswath mask from frequency B only is applied to remove partially focused regions (in main_diff_ms_band method and when preprocessing is enabled in unwrap.py).
• In some areas, the locations of partially focused regions differ between frequency A and B. As a result, pixels that are invalid in frequency A but valid in frequency B remain in the differential interferogram. These residual pixels introduce phase inconsistencies that propagate into the unwrapping step, leading to phase jumps (above figure).

Solution

• When computing differential interferogram between A and B, we read both subswaths from A and B and mask out the partially focused areas.

Updated Ionosphere

RSLCs used for this analysis
reference_rslc_file: NISAR_L1_PR_RSLC_006_148_A_022_2005_QPDH_A_20251202T105655_20251202T105730_X05009_N_F_J_001.h5
secondary_rslc_file: NISAR_L1_PR_RSLC_007_148_A_022_2005_QPDH_A_20251214T105655_20251214T105730_X05009_N_F_J_001.h5