mbio: expose Reson 7k3 SonarSettings (record 7000) via new mb_sonarsettings(), surfaced in mblist

[epifanio]

Summary

The Reson 7k3 reader (mbsys_reson7k3) already parses the 7000 Sonar Settings record, but the only settings reachable through the format-independent API are transmit_gain / pulse_length / receive_gain (via mb_gains). The rest of the recorded settings — frequency, sample rate, transmit pulse width, power & gain selection, absorption, spreading, sound velocity, and tx/rx beamwidths — are needed for backscatter processing (removing the time-varying gain, and applying transmission-loss / insonified-area corrections) but are currently locked in the system-specific struct.

This PR adds a small, purely additive accessor to surface those recorded values, and wires the most useful ones into mblist.

API

mb_sonarsettings() mirrors the existing mb_gains():

• new function pointer mb_io_sonarsettings in struct mb_io — NULL by default, so formats that don't implement it return MB_ERROR_BAD_SYSTEM and no other format readers are touched;
• generic wrapper mb_sonarsettings() in mb_access.c (prototype in mb_define.h);
• implementation mbsys_reson7k3_sonarsettings() returning the recorded 7000 values, registered in mbr_reson7k3.c.

mblist

mb_get_raw() learns about format 89 (Reson 7k3) through mb_sonarsettings(), so the existing raw output codes now work for Reson 7k data:

code	value
-O.a	mean absorption (dB/km)
-O.r	sampling rate (Hz)
-O.L	transmit pulse length (µs)

(transmit power and receiver gain are already available via -O.T / -O.t through mb_gains). Man page updated.

Note on proprietary data

This only surfaces values the sonar wrote into the file (the 7000 record MB-System already parses). It applies no vendor gain/TVG model — there is nothing proprietary here.

Testing

• libmbio and mblist build cleanly.
• Verified against real R/V Hugh R. Sharp Reson 7125 data (format 89), e.g.:

  $ mblist -I surveyp.mb89 -O"T.a.r.L"
  2015/06/19/19/42/06.160208   44.00   34483    100
  ...
  

  (time, absorption dB/km, sample-rate Hz, pulse-length µs)

Possible follow-ups

The accessor already returns frequency, spreading, sound velocity, and tx/rx beamwidths; these could be surfaced via additional mblist codes, and mb_sonarsettings() could be implemented for other systems that record equivalent settings (e.g. Kongsberg .kmall, Reson 7kr).