remove `__add__` operator

[dev-dull]

__add__ was written to try to make _DDB more user friendly. When it was written, the DDB abstraction around _DDB didn't yet exist (to put it another way, what is now _DDB was once called DDB), so at the time, it made (some) sense to try and make things friendly with the + operator. Since _DDB is no longer meant to be used directly, we can change this kludgy code to a traditional function call instead.

Code refs:

• PyXIE/ddb.py

  Line 173 in 39a0c12

  self[id] + user_data

• PyXIE/ddb.py

  Lines 230 to 237 in 39a0c12

  	def __add__(self, user_data):
  	now = time()
  	while now in self:
  	sleep(0.1)
  	now = time()
  	self[now] = user_data
  	self._cleanup()
  	return self