tethys_equipped face North by default

[chapulina]

• Part of Document / update coordinates and units #48
• Builds on top of Spawn vehicle at requested orientation #143

This PR updates the tethys_equipped model so that it faces North by default, i.e. zero orientation means it faces North, to match what the controller expects. Before this PR, the model was facing West by default.

Before this PR	After this PR

Summary:

• ⚠️ tethys_equipped faces North / +Y, but tethys by itself still faces -X / West
• The state message has been updated to publish pos and RPH in NED (see tests)
• ⚠️ The model's base link (canonical link) still faces the model's -X, which is the world's -Y at pose zero

[arjo129]

Thanks for getting this in! I like the suffixing of coordinate systems to variable names, gonna start doing that. This looks good to me.

[chapulina]

Thanks for the review, @arjo129 . I've addressed all of your comments. This PR builds on top of #143 because without that the spawn test would fail. I think it's best to merge that one first so that it's easier to review.