Added support for visibility distance/barometric pressure conversion units

[Darumin]

Attempt to fulfill #325 by adding conversions in measurables.py for pressure ("Hg) and visibility (kms/miles), also added unit tests to go with them.

[csparpa]

Hi @Darumin so far it looks nice, thanks!

A couple of improvements:

One - As you've already coded the core conversion logic, I would now advice to expose it through new methods of the Weather class
These might be the very similar to the wind() method:

def wind(self, unit='meters_sec'):
  ...

Something like:

def pressure(self, unit='hPa'):
  ...
  
  
def visibility_distance(self, unit='m'):
  ...

Please, only make sure that the provided default units are from the International Decimal Metric System (m & Pa)

Two - In order to let the world know and use your excellent work, why don't you also update the technical docs here and the quick code recipes https://github.com/csparpa/pyowm/blob/develop/sphinx/v3/code-recipes.md?

Thanks a lot!

[Darumin]

I added the matching methods to Weather class. I also refactored each use of self.pressure to self.press to avoid collision with the new pressure method.

You'll also see new recipes and utilities usage examples, as requested. Let me know if there's anything else that needs clarity.

[csparpa]

Hello @Darumin thanks for the contribution, it's on the develop branch

[Harmon758]

@csparpa This doesn't seem to have actually been merged?
I can't find the changes in either the develop or master branches, nor in v3.1.1.

Also of note, there's technically a breaking change here, with Weather.pressure being a method instead of an attribute now.

[csparpa]

@Harmon758 I probably have messed the merge up on my local setup :-S
I will take a look into that as soon as possible - thanks for pointing the breaking change out

[csparpa]

@Darumin @Harmon758 I've merged this.

As the newly introduced pressure() function was replacing the already existing pressure attribute of Weather objects, I had to rename the function to prevent existing pyowm clients from breaking.

Now the funciton is called barometric_pressure