Enable Sentinel-1 Extra Wide (EW) SLC compatibility

[abradley60]

Enable Sentinel-1 Extra Wide (EW) SLC compatibility

Rationale

These changes are needed to enable Sentinel-1 EW burst compatibility with S1 reader. It is an important resource in polar regions, where processing EW SLC data can enable new scientific insights. Like the IW mode, Sentinel-1 EW SLC's contain information at the burst level which is useful for processing when complex data is required. The changes extend the software's capabilities and should not impact any existing behaviour. These changes are a pre-requisite for enabling EW burst processing in other software tools such as https://github.com/GeoscienceAustralia/RTC.

Changes Overview

• Sensor mode detection: Logic now handles both iw and ew modes, inferred from the SAFE filename. The main structural difference is subswath count (3 for IW, 5 for EW).
• Mid-swath logic generalised: EW3 is the reference subswath for EW mode (analogous to IW2 for IW mode). S1BurstId.from_burst_params now computes burst IDs for either mode using the appropriate timing constants and reference subswath offset.
• EW timing constants added: T_beam_ew, T_pre_ew. IW related timings have kept original namings (e.g. T_beam).
• New ew3_mid_range field on Sentinel1BurstSlc: An Optional[float] field appended as the last dataclass field for backwards compatibility. EW bursts populate it and leave iw2_mid_range as None. IW bursts do the reverse. The bistatic_delay property branches on whichever is set.
• New constants.py module: Holds SENSOR_MODE_SUBSWATHS and SENSOR_MODE_MID_SWATH as a standalone module to avoid circular imports.
• CLI updated: s1_info gains an --ew flag.
• Backwards compatibility: No existing IW behaviour should be changed. New burst_from_xml argument ew3_annotation_path is keyword-only with default None; ew3_mid_range is the final dataclass field.
• EW tests and test data added