Provide a public method for iradon_filters

[hmaarrfk]

Description

Continuation of #3099

Not sure why
#4950 was failing.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[emmanuelle]

was it a bug which you corrected? I don't understand why f is zero on even values of indices, this does not correspond to Kak and Slaney I think.

[emmanuelle]

ok I took a look at one of the previous PRs and apparently it's not the classical ramp filter which is used here. I'll try to understand better which filter is used here. Did you just make this change for the sake of a clearer syntax or is there a difference?

[hmaarrfk]

I think at the time, the change was introduced for cleaner syntax.

[hmaarrfk]

The "fixes" for the filter performance were merged in a pervious PR.

[rfezzani]

Your current refactoring of n calculation is different from previous one:

>>> size = 10
>>> n1 = np.arange(0, size / 2 + 1, dtype=np.int)
>>> n2 = np.arange(size / 2 - 1, 0, -1, dtype=np.int)
>>> n = np.concatenate((n1, n2))
>>> n[1::2]  # This PR refactoring
array([1, 3, 5, 3, 1])
>>> np.concatenate((np.arange(1, size / 2 + 1, 2, dtype=np.int),
                    np.arange(size / 2 - 1, 0, -2, dtype=np.int)))  # Previous version
array([1, 3, 5, 4, 2])

I don't know which version is correct, but there is a difference...

[jni]

I'm not familiar with these methods but the code looks good to me. I'll let others more familiar with radon review/merge but it'd be good to get this in.

@hmaarrfk could you please elaborate more on the motivation for the PR in the description? When making release notes, it's quite annoying to have to go back 7 links to figure out why this PR was made. (The PR you link to is also described as "continuation of...")