Blend speed up

[jonathanMweiss]

this commit tries to address issue #12
I've reduced loop iterations while creating the coefficient matrix and b vector (by Using NumPy/Scipy operations).
This reduced the runtime on my machine from 4.5~ seconds on average to 3.6~ seconds on average.

Furthermore I've changed the function which solves the linear problem from pyamg.solve to Scipy.sparse.linalg.spsolve.

All changes together reduces runtime on average from 4.5~ to 0.5~ seconds.

All tests were done with the test() function in file poissonblending.py, time measure without saving the image.

[fbessho]

Hi @mcboomy - Thank you so much for contributing to PyPoi. The run time indeed reduced significantly on my machine too. Great job!!

[jonathanMweiss]

I haven't received a notification and forgot about it. Thank you for merging!