Version 0.8

• new observation types equivalent (potential) temperature, mixing ratio,
  absolute humidity, and (saturation) vapor power
• option horizon and use_center for $daylight and .daylights
• option timestamp for $daylight to simplify for loops
• get correct temperature and barometer for calculating sunrise and sunset in $daylight
• bug fix in energy_integral
• more bug fixes
• property length for timespans