Radon transform: Redefine projection center for sinograms.

This definition is chosen because it is simple to express in the
documentation. No changes in accuracy are to be expected, but comparing
sinograms and reconstructions before and after this commit will give
different results in the cases where ``circle=False`` for ``radon`` or
``iradon``.
Radon transform: Document rotation axis location.
radon: Reduce duplication; simplifications.
radon: Correct docstring of order_angles_golden_ratio.
Radon transform: Include boundary in reconstruction circle.

A test criterion needed to be relaxed slightly to have tests still
passing. This is ok, as the reconstruction circle is now larger, meaning
larger errors should be expected. Moreover, the test in question uses
random data, and changing the seed causes greater changes in accuracy
than the amount the test criterion was relaxed by.
Radon transform: PEP8 fixes.
iradon: Clean up filter code.
iradon: Do not suppress 0 frequency for filter=None.
Merge pull request #654 from josteinbf/radon-square-center

Define rotation axis center for Radon transforms with inverses.