Merge pull request #596 from josteinbf/radon-projection-center

Radon transform: Improve tests and address bugs in definition of image and projection center.
Add SART tomography reconstruction to radon_transform.
Make iradon_sart visible in the transform package.
Add tests for transform.iradon_sart.
Implement projection shifts.
Tests for the projection_shifts functionality of iradon_sart.
Fix docstrings for transform.iradon_sart with subroutines.
transform.iradon_sart: Clean up code for ordering projections.
iradon_sart: Add wikipedia reference to Kaczmarz method.
iradon_sart: Clarify how constants are chosen.
Tests for iradon_sart: Robust path handling.
iradon_sart: Clean up cython code to minimize python calls.
iradon_sart: Improve argument checking.
iradon_sart's cython exts: Use typed memoryviews, not ndarrays.
iradon_sart: Reduce code duplication in interpolation.
iradon_sart: Remove needless memoryview/ndarray conversion.

image_update is not manipulated in sart_projection_update and the
conversion to memoryview will happen when it is passed to the
subroutines anyway.
iradon_sart: Add Kaczmarz reference and reformat citations.
radon_transform: Declare encoding.
iradon_sart: Order angles using a golden ratio approach.
iradon_sart: Add test for order_angles_golden_ratio.