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.
Fix ratio scaling in SLIC
Remove unused 'ratio' argument from _slic_cython
Precision issues appear to have vanished
Adding Octagon structural element
Removing circular import
Replace 'ratio' kwarg with 'compactness' in SLIC
Do not use 'ratio' in test_slic.py either
Correcting grammatical typo
Making the doc more explicit
Added star structural element
Minor doc changes
Test for Octagon