Add support for the extended Polygon geometries

[gbroccolo]

The PR contains the definitions of the extended geometries for Polygon type. As made for the previous extended geometries, basics methods for the new types have been defined to reduce the extradimensions to 2D objects in order to be integrated with the rest of the repo.

NOTE: Additional Line types have been defined to complain the AsSegment method of the extended Polygons in order to obtain segments of 3D, 2D+1D and 3D+1D points.

[coveralls]

Pull Request Test Coverage Report for Build 511

• 179 of 213 (84.04%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 56.184%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
polygonm.go	25	29	86.21%
polygonms.go	26	30	86.67%
polygons.go	26	30	86.67%
polygonz.go	25	29	86.21%
polygonzm.go	25	29	86.21%
polygonzms.go	26	30	86.67%
polygonzs.go	26	30	86.67%
line.go	0	6	0.0%

Totals
Change from base Build 499:	0.6%
Covered Lines:	6637
Relevant Lines:	11813

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💛 - Coveralls

[gdey]

LGTM