Fix division by scalar for `Twist2` classes

[btalb]

Division by a scalar works for Twist3 classes, but throws this error for Twist2 classes:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-05a7052c8927> in <module>
----> 1 t / 1                                                                                        

~/qut/repos/spatialmath-python/spatialmath/twist.py in __truediv__(left, right)
    283     def __truediv__(left, right):  # lgtm[py/not-named-self] pylint: disable=no-self-argument
    284         if base.isscalar(right):                                                             
--> 285             return Twist3(left.S / right)                                                    
    286         else:                             
    287             raise ValueError('Twist /, incorrect right operand')            

~/qut/repos/spatialmath-python/spatialmath/twist.py in __init__(self, arg, w, check)
    328         if w is None:                                                                        
    329             # zero or one arguments passed 
--> 330             if super().arghandler(arg, check=check):
    331                 return
    332             elif isinstance(arg, SE3):                                                       

~/qut/repos/spatialmath-python/spatialmath/baseposelist.py in arghandler(self, arg, convertfrom, check)
    182         elif isinstance(arg, np.ndarray):
    183             # it's a numpy array                                                             
--> 184             x = self._import(arg, check=check)
    185             if x is not None:  
    186                 self.data = [x]

~/qut/repos/spatialmath-python/spatialmath/twist.py in _import(self, value, check)
    357         elif base.ishom(value, check=check):                     
    358                 return base.trlog(value, twist=True, check=False)
--> 359         raise TypeError('bad type passed')
    360                  
    361     @staticmethod                         

TypeError: bad type passed

The error is caused by __truediv__() in the BaseTwist class returning a Twist3() instance when divided by a scalar, rather than a type corresponding to the left param.

This PR fixes this.