Fix incorrect QNH in spoken ATIS when using live weather fetch

Fix incorrect QNH in spoken ATIS when using live weather fetch

When using live weather fetch, the QNH should be obtained from
environment/metar/pressure-inhg.

See: http://sourceforge.net/p/flightgear/mailman/message/35037125

Add new method getQnhInHg to ATISInformationProvider and its implementations
to avoid rounding errors converting from hPa back to inches in ATIS reports.

The CurrentWeatherATISInformationProvider (used when live weather fetch is
not in use) continues to use the property environment/pressure-sea-level-inhg.
This produces the incorrect QNH at airports significantly above sea level
but this needs fixing elsewhere to calculate the correct QNH.