transform.radon: Add test to verify the projection center.

This test is designed for issue gh-592.
transform.radon: Add testcases for rectangular input arrays.
transform.iradon: Add tests for center of projection.

This is a test designed for resolving gh-592.
transform.radon: Remove unneccesary matrix inverse.
transform.radon: Use correct padding for rectangular images.
transform.radon: Consistent definition of center of array.

The center is now defined as shape[i] // 2.
radon tests: Refactor test for circle reconstructions.
transform.iradon: Redefine slice and projection center.

These changes should match those made to radon.
test_radon_transform: Add helper functions.
transform.iradon: Refactoring for shorter functions.

Will facilitate testing.
test_radon_transform: Test sinogram conversions.
test_radon_transform: Refactor and improve test_radon_iradon.

Aside from refactoring, the Shepp-Logan phantom is now used as it is a
more challenging test object.
test_radon_transform: debug option for test_iradon_minimal.
radon_transform: Stylistic changes.
test_radon_transform: Refactor tests and make them stricter.
test_radon_transform: Change test criterion for iradon_minimal.
test_radon_transform: Change test criterion for sinogram_circle.
transform.radon: Robust determination of center of projection.
transform.iradon: Correct determination of center of projection.
test_radon_transform: Clean up imports.