Revert "Raise an IndexError when slicing a single-segment PS"

This reverts commit b12edb9.

Author: thangleiter

filter_functions/pulse_sequence.py

@@ -395,8 +395,6 @@ def __getitem__(self, key) -> 'PulseSequence':
         new_dt = np.atleast_1d(self.dt[key])
         if not new_dt.size:
             raise IndexError('Cannot create empty PulseSequence')
-        if len(self) == 1:
-            raise IndexError('invalid index to scalar (single-segment) PulseSequence')
 
         new = self.__class__(
             c_opers=self.c_opers,