fix have_sunrise_and_sunset

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add is_summer and is_winter

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m doc + add equinox prediction

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add solar_prediction and use it for future_past_equinox and future_past_elevation

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add solstice prediction

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add solar elevation derivative prediction

Signed-off-by: Mattias Andrée <maandree@operamail.com>