Add medium type, as new property (air, water, rock...)

See merge request !265

EM-Processes which are in a different media than Air are now provided.

Update documentation

See merge request !263

Merge branch '222-distinguish-between-alternative-interaction-processes-and-competing-processes' into 'master'

Resolve "Distinguish between alternative interaction processes and competing processes"

Closes #222 and #162

See merge request AirShowerPhysics/corsika!269