Remove sptensor.extract()

[dmdunla]

I proposed that we remove sptensor.extract, as the sptensor.__getitem__ functionality should support all of the use cases that we need. Although we proposed to remove sptensor.extract in #160, it is still being used.

Here are the steps that need to be addressed:

• Replace use of sptensor.extract with use of sptensor.__getitem__. Note that the output of extract for sparse tensors with 1 nonzero returns numpy.ndarray, whereas __getitem__ was recently changed to return a scalar in this case.
• Replace use of sptensor.extract in sptensor.__getitem__.

The way that the MATLAB TTB sptensor accessor analog to __getitem__ (i.e., subsref) works is to allow users to include an additional input parameter, 'extract' to extract the result as a vector. I think we should stick with the more numpy-like approach and post process the output of __getitem__ when needed.