Add unfold function for point arrays

[parrangoiz]

Description of changes:
Added an unfold function to the Polygons module. This can be used to make it easier to construct polygons that have a mirror symmetry. I have found this utility quite useful in my work so I figure I'd add it to the package.

Issue #, if available:
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[gpeairs]

Looks useful. Can I suggest some modifications so that the interface follows the same pattern as reflect_across_line? In other words:

• Don't restrict the type of dir, so that it can be a Point or an angle (the Reflection constructor accepts either)
• Add a keyword argument through_pt=nothing that also gets passed to the Reflection constructor, so that the line across which points are reflected can be defined with a direction and a point other than the origin
• Add a method that accepts two points defining a line across which the points are reflected (there's a Reflection constructor for this too, or this can call the method above)

[parrangoiz]

@gpeairs I like the suggestion, done.

[parrangoiz]

Do I need to add it explicitly to the docs or do we have autodocs for certain modules?

[gpeairs]

No autodocs, everything has to be added explicitly for now.

[parrangoiz]

Ok added it to the polygon API section but let me know if you think there's a better place for it.

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[gpeairs]

Looks good, thanks!