Both `BaseStage.backlash_steps` and `BaseStage.axis_inverted` (and the subsequent redefinitions of `axis_inverted` have default values that are dictionaries. Mutable default values can cause strange things to happen, especialy if more than one instance of an object is created. I observed this because it caused a problem in the unit test suite (#750). While the bug only appears when a specific LabThings feature is enabled, I think it would be good practice to fix it here. I'll open an MR shortly.