Add altimeter units and leave unit conversion up to the user

[aeharding]

@drmrbrewer and @clioh, if you have a moment I would appreciate a quick review!

Specifically, I wasn't sure whether to specify AltimeterUnit's value as "mb" (millibars) or "hPa".

What do you prefer?

[clioh]

The PR LGTM.

I don’t work in that part of the world so I’m not sure what the preferred terminology is. Looks like the internet uses them pretty interchanbly so I’d say go for whichever you like more

[drmrbrewer]

I think that hPa is more common in all the weather-related APIs I have used... in fact I can't remember it ever being documented as mb (not that it would be wrong to do so)... hence hPa is preferable IMHO! Seems like this is as described here too... i.e. hPa preferred though mb not wrong (and still used in some places)... Pa is the SI unit for pressure so it seems somewhat old-fashioned to be referring to millibars.

[aeharding]

Great, thanks for the feedback! I'll keep it as it is then :)