Optional altitude should be supported

[SDeans0]

The RFC specifies that there must be an optional altitude value - currently this implementation does not support that.

See Section 4

An OPTIONAL third-position element SHALL be the height in meters above or below the WGS 84 reference ellipsoid

This is fairly easy to fix, but may be a breaking change if any applications are relying on tuple unpacking for the coordinates as they require an extra parameter.

The RFC doesn't make any mention of whether you should allow mixtures of 2d and 3d points in collections. It seems that it would be common sense that you shouldn't, but since that's not specified in the RFC, it probably shouldn't be enforced by the library.

[SDeans0]

@folt I've got a PR here ready for review, could you take a look?

[SDeans0]

We could potentially make it backwards compatible by adding a 3DCoordinate class to support 3D and then replacing the coordinate with union of Coordinate, 3DCoordinate.

[folt]

that's cool, i'll definitely check it out soon

[SDeans0]

Hey @folt, do you have any further thoughts on the altitude PR？