Enable use with 3d or other custom point types

[jfirebaugh]

No description provided.

[mourner]

Would be nice to also demonstrate the custom use in tests.

[jfirebaugh]

Added a test and README example. Thoughts @springmeyer @artemp?

[mourner]

Needs to be updated for compatibility with for_each_point and envelope.

[jfirebaugh]

Updated for_each_point and fixed a bug in multi_polygon_t. With envelope, we maybe need to reinvent another feature of boost::geometry, which is generalizing algorithms across the dimensionality of their input. You'd expect envelope to calculate the 3d envelope if given 3d input geometry.