Fix pressure in dark sky and openweathermap and add pressure utility #21210
Dark sky and open weather map both displayed pressure in hPa for imperial unit system, although front end displayed inHg as units. This PR checks unit system and converts to inHg if necessary.
Pull request in home-assistant.io with documentation (if applicable): NA
Example entry for
Upon further reflection, pressure in
I couldn't find any existing usages of unit system like this.