Bug with lazy variance and masked arrays

[pelson]

A very specialised bug with the variance calculation when producing masked arrays (smaller arrays, and accessing ndarray work fine):

>>> import biggus
>>> import numpy as np
>>> 
>>> arr = np.empty((10, 400, 720), dtype=np.float32)
>>> r = biggus.var(arr, axis=1)
>>> 
>>> r 
<_Aggregation shape=(10, 720) dtype=dtype('float32')>
>>> r.masked_array()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "biggus/__init__.py", line 1555, in masked_array
    result, = engine.masked_arrays(self)
  File "biggus/__init__.py", line 448, in masked_arrays
    return self._evaluate(arrays, True)
  File "biggus/__init__.py", line 442, in _evaluate
    ndarrays = group.evaluate(masked)
  File "biggus/__init__.py", line 428, in evaluate
    raise Exception('error during evaluation')
Exception: error during evaluation
>>> Exception in thread <biggus.StreamsHandlerNode object at 0x1949850>:
Traceback (most recent call last):
  File "lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "biggus/__init__.py", line 277, in run
    self.output(self.process_chunks(input_chunks))
  File "biggus/__init__.py", line 305, in process_chunks
    return self.streams_handler.process_chunks(chunks)
  File "biggus/__init__.py", line 1341, in process_chunks
    result = self.finalise()
  File "biggus/__init__.py", line 1501, in finalise
    chunk = super(_VarMaskedStreamsHandler, self).finalise()
  File "biggus/__init__.py", line 1459, in finalise
    array.shape = self.current_shape
ValueError: total size of new array must be unchanged

[pelson]

This is still a live issue on master. I've now got somebody who has the problem (not just theoretical anymore) so I should get a good chance to look at this.