Radon transform: Improve tests and address bugs in definition of image and projection center

This pull request has three (related) goals:

* Fix issue #592. Accuracy of reconstructions has improved, and there are less artifacts than before.
* Refactor and improve tests for `radon` and `iradon`. In particular, the old tests were not sufficiently strict. Futhermore, many of the tests are now parametrized, as I found this to be very useful when researching #592.
* Refactor `radon` and `iradon` to make the code more readable. The lack of readability is probably an important reason for why the code was buggy in the first place.

I believe this is essentially ready for merge. When this is merged, test for PR #584 will probably start failing, and that is a good thing. It would be nice to merge this relatively quickly, so #584 can be updated.

`radon` and `iradon` could certainly use more cleanup to improve readability, but I think it is better to get back to that in future PRs.

Last but not least: Anyone who has used `radon` and `iradon` for any serious work ought to rerun their code with this version to look for changes. The errors of the old code can be subtle to someone not very familiar with tomography, and the artifacts can be very hard to distinguish from the effects of having too few projections.