Fix fatal error when combining segmentlists with invalid values

An expression like this with a binary operation between a segmentlist on the right hand side and certain incompatible values on the left hand side would crash a Python 3 interpreter:

>>> from glue.segments import segmentlist
>>> None | segmentlist()
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: None
Fatal Python error: deallocating None

Current thread 0x00007fffa0e7e340 (most recent call first):
Abort trap: 6

Several such expressions are evaluated in test_segmentlist.test_typesafety in segments_verify.py, so this would cause the unit tests to fail.

The reason was that the C implementations of segmentlist.__and__, segmentlist.__or__, and segmentlist.__sub__ assumed that the left hand side argument was an instance of segmentlist. However, this is not necessarily the case as Python will try both v.__or__(v, w) and w.__or__(v, w). Since segmentlist.__and__ was trying to create a new instance of the class of the left hand side operand, it was basically trying to instantiate NoneType, which is all kinds of bad.

The solution is to use the class of the left hand side for the class of the return value if the left hand side is an instance of segmentlist, or the class of the right hand side otherwise.

Fixes #4 (closed).