fix zero step size bug in unwrap_error_phase_closure

[yunjunz]

Description of proposed changes

• unwrap_error_phase_closure: replace np.rint() with np.ceil() to ensure non-zero step size, which could happen for long time-series with highly redundant networks (A problem about calculating the number of non-zero closure phase #1025).

Reminders

• Fix A problem about calculating the number of non-zero closure phase #1025
• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
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