Estimate sound speed

## Summary
Use depth/pressure, water temp, and salinity to estimate sound speed.

## Additional information
- Could accept relevant variable values from user input, or telemetry, or a combination (most ROVs don't have salinity/conductivity sensors)
- Would make sonar data more accurate
- May need to add `v_sound` to logging somehow:
- Ping360 Ping Viewer integration uses a quantised scan range (highest precision is integer meters), so when analysing it would be possible to programatically determine the `v_sound` that was used for a given ping by the sample period and number of samples if necessary - I could make a python example of how to do this as part of a more general analysis example, or as a separate 'advanced' one.
- Ping1D has no sample period property in the profile messages (or in the protocol at all), but actively uses `v_sound` for calculations, so perhaps Ping Viewer could send a `get v_sound` request after every `set v_sound` command so that it's appropriately logged.
- Likely best to use the UNESCO equation, but it may be sufficient/acceptable to use the Coppens* equation (a bit older and not quite as accurate, but has fewer terms and is similar in generality) - both equations (and a few others) detailed [here](http://resource.npl.co.uk/acoustics/techguides/soundseawater/underlying-phys.html).

*(My) Python example code for Coppens here:
```python3
def coppens(D,S,T):
''' The Coppens speed-of-sound in sea-water equation [m/s].

See: resource.npl.co.uk/acoustics/techguides/soundseawater/underlying-phys.html

'D' is depth in meters [0-4000].
'S' is salinity in parts-per-thousand (ppt) [0-45].
'T' is temperature in degrees Celsius [0-35].

All input parameters can be a single number or numpy array of numbers.

'''
D = D / 1000 # ensure copy to not modify possible input array
t = T / 10
t2 = t * t
t3 = t2 * t
dS = S - 35
c_0St = 1449.05 + 45.7*t - 5.21*t2 + 0.23*t3 + (1.333 - 0.126*t + 0.009*t2)*dS
c_DSt = c_0St + (16.23 + 0.253*t)*D + (0.213 - 0.1*t)*D*D + (0.016 + 0.0002*dS)*dS*t*D
return c_DSt
```

No files or tests are named. Start by reviewing the Ping360 Ping Viewer and Ping1D integration points alongside the provided Coppens example; define how depth or pressure, temperature, and salinity arrive, how v_sound is logged, and how the chosen equation and resulting sonar accuracy should be verified.

Written by the indexing model from the issue text.