planet_position(): optionally sacrifice accuracy for speed

* planet_position(): optionally sacrifice accuracy for speed
  * added optional dummy variable ltime to planet_position()
  * if set to .false., the light-time correction is not applied
  * this saves ~50% CPU time at the cost of a bit of accuracy
planet_position_la() calls planet_position(ltime=.false.) for planets

* planet_position_la() calls planet_position(ltime=.false.) for planets
  * does not take into account light-time correction to gain ~50% CPU time
Added optional dummy variable ltime to riset()

* Added optional dummy variable ltime to riset():
  * pass it on to planet_position() to save ~50% CPU and lose a bit of accuracy
  * pass it on to riset_ipol() for the same reason
Set ltime=.false. by default in riset

* Set ltime=.false. by default in riset:
  * call riset(ltime=.true.) to account for light-travel time
  * difference in rise/set times is in the order of seconds
  * difference in rise/set azimuth is in the order of 0.01 degree
  * difference in CPU time is about a factor of 2