Feature/coherence matrix time axis clean

[huchangyang]

Description of proposed changes

This PR adds an optional continuous time-axis view for coherence matrix plotting.

The new --time-axis option in plot_coherence_matrix.py displays the coherence matrix using acquisition dates as a continuous time axis instead of image indices. Each acquisition is represented by a date-centered cell. Internal cell boundaries are placed at the midpoint between adjacent acquisition dates, while the first and last cells are extended outward by the adjacent acquisition spacing.

This makes temporal gaps in the interferogram network visible in the matrix view. The existing upper/lower triangle convention is preserved: the upper triangle shows used interferograms, and the lower triangle shows all available interferograms.

This PR also adds coherenceMatrixTimeAxis.pdf to plot_network.py outputs for coherence datasets.

Depends on: PR1 (feature/coherence-matrix-split-windows-clean)

Reminders

• Fix #xxxx
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