Updating visibility to version 1.2. Biggest changes are the handling of low clouds/mist.
Updating visibility to version 1.2. Biggest changes are
the handling of low clouds/mist.

Also the two different visibility algorithms are split to
two separate functions, to make the control flow easier to
grasp. This change accounts for the majority of the line
changes in the commit.

fmidev/himan,fmidev/himan,fmidev/himan