Add a pose relation option to suppress error along z-axis

[mitchallain]

As alluded to in my comment on #577, I have added a new pose relation option called xy_trans_part which suppresses error along the z-axis.

[MichaelGrupp]

Thanks for the PR. Projection should be a preprocessing step of the trajectory in my opinion, not a property of the metric. This makes it more flexible. Additionally, xy is not the only plane that one might be interested in (e.g. for visual SLAM people xz could be more common). Adding a pose relation for each of the three possible planes in both metrics is then maybe not so ideal.

I implemented a generic projection feature that would produce the same result as your implementation when used with evo_ape ... --project_to_plane xy. And you can also use it in evo_traj independent of a metric. Will probably be merged and released soon:

https://github.com/MichaelGrupp/evo/pull/607

[mitchallain]

Nice, even better! I'll go ahead and close this