Added depth and pressure, updated README

[808brick]

Hello, I have used the C++ version of this KellerLD module that is used for microcontrollers:
https://github.com/bluerobotics/BlueRobotics_KellerLD_Library/

That library features calculations to convert the pressure obtained from the sensor to Depth and Altitude readings. I was hoping this feature could be added to this module, as we were utilizing this python module for use with Jetson Nano's, which have a GPIO pinout to read in the values from the BlueRobotics sensor.

I made modifications to the code, and tried to keep similar naming conventions / styles to that of your Python setup, as well to reference the way you have does the calculations in the C++ version of this module.

Things that were added/changed:

• sensor.pressure() function can now take in a string of 'Pa', 'bar', 'mbar', to change the unit of pressure returned from the sensor. Defaults to 'bar', and will throw an assertion exception if an invalid unit is given.
• New sensor.depth() and sensor.altitude() functions which basically copies the functionality of the C++ version of this module.
• Updated README so people are aware of the added functionality mentioned in the previous 2 points
• Added a sensor.set_fluid_density() function, similar to that of the C++ version.
• Updated example.py to include the new functions, and to break the while loop properly when Ctrl + C is pressed.

The modified code was tested on a Jetson Nano (by running the modified example.py script, and changing the pressure units) and seems to perform as expected.

I'm open to suggestions / changes to better match your formatting preference, but figured this would be useful for people in the future so they do not have to do their own conversions / calculations, especially if they are in a situation like me where they migrate from the functionality in the C++ microcontroller module, to this Python version.

-Raymond

[ES-Alexander]

Hi, just saw this! Thanks for submitting these changes :-)

Mostly looks good, just a couple of suggestions / things to consider :-)

[ES-Alexander]

This should probably use set_fluid_density (e.g. in case someone changes the function to include logging later), and should probably also have a parameter in the __init__ definition (e.g. def __init__(self, bus=1, fluid_density=1029):) so it's settable directly on object instantiation.

[ES-Alexander]

This is a bit confusing, because the fluid_density parameter has no default value - the "default" mentioned in the docstring is instead a default from class instantiation (which is outside the scope of this function, and likely isn't relevant to mention here).

It may also be worth adding a few string-based defaults, either just 'fresh'->997 and 'sea'->1029, or could make use of wikipedia's salinity levels by water type overview. Not essential, just could be nice to have (although those levels could also be implemented separately in a Water class or something, which may be more appropriate).