Adds swirl function

[paniash]

This PR adds a swirl as a standard function.

Relevent issues: #133

[johnnychen94]

To make it a standard library function, we need to:

• polish the API:
  • should we take the scikit-image version, or invent our own?
  • The example swirl function uses the center point as the swirl center x0, I think we can let users decide where the swirl center is.
• add tests
• add documentation

You can take a look at imrotate function as an example. For tests purposes, we can use ReferenceTests.

[johnnychen94]

Is there any plan to finish this PR?

[paniash]

Hi! Sorry, life's gotten in the way. Please feel free to go ahead. :-)

[johnnychen94]

Thanks. I'm closing this in favor of #146