Need ability to convert 3D coordinates

`Proj::convert` assumes 2D coordinates, and passes `z: 0.0` into `proj_sys` (which handles 3D just fine).

This is insufficient for conversions to/from (e.g.) ECEF coordinates, where the z coordinate is critical even for locations on the ground.

Then need to access the private `c_proj` makes it impossible to define a trait with a minimally-modified `convert` method which takes a 3D point, passes all three values in/out of proj_sys, and returns a 3D point.

Some provision for 3D coords within this crate would be extremely helpful please :-)

Start by reading `Proj::convert`, the call into `proj_sys`, and the private `c_proj` access described in the issue. Determine how a 3D point should be accepted and returned while preserving existing 2D behavior. Done means conversions can pass all three coordinates to and from `proj_sys`, including ECEF use cases, without requiring external access to private internals.

Written by the indexing model from the issue text.