Export headers for DepthTraits and convert

This PR exports `DepthTraits<T>` and `convert` for `xyz` PC in two separate headers to avoid code duplication in other places, in particular in an upcoming PR following https://github.com/ros-perception/image_pipeline/pull/104.

The `convert` function is also extended to support to allow for setting `range_max` for invalid (when there's no "echo"/"return") pixels.

The `CMakeLists.txt` is also slightly cleaned up.