Switching to Unix ends-of-lines.
Add a "silent" property to DT::E::SR objects and provides the flag for polar night / polar day / day and night.
The new functions, which enable users to use a silent DT::E::SR object without losing
the fact that the sun "never rises" or "never sets".
The three boolean methods should always be silent, even if the DT::E::SR object is not.
A bit more documentation for the "is_polar_day" and "is_polar_night" methods.