Added a landing position offset value and method.
Populate it by looking for `tag_landing` in a ship model.
Use the new ship landing position offset value.
Cleanup passing the ship pointer around to where it's no longer used and assert that it's valid where it is.
Defunct stuff.
New starting ships
Replace the amphiesma high detail model with one which has the tag_landing built into it.
Merge remote-tracking branch 'nozmajner/start_ships'
Merge remote-tracking branch 'fluffyfreak/tag_landing_issue_2656'
Merge branch 'master' of https://github.com/pioneerspacesim/pioneer into gas_giant_split