Modified _orbits.py to fix orbit period comparison error.
Modified _orbits.py to fix orbit period comparison error.

orbit period defaults to a string, which in the latest pandas
will not be automatically converted to a timedelta at compariton time
added instantiation of a pandas timedelta object at self.orbit_period definition
This should allow safe comparison to other timedelta objexts wherever self.orbit_period is used

rstoneback/pysat,jklenzing/pysat