EMSUSD-946 axis in import UI and command

[pierrebai-adsk]

• Add -upAxis (-upa) and -axisAndUnitMethod (-aum) flags to the base import command.
• Document the new flags in the import command read-me file.
• Add upAxis and axisAndUnitMethod tokens to the import job args tokens.
• Add the rotateScale, addTransform and overwritePrefs tokens to the import job args token.
• Add the upAxis and axisAndUnitMethod data to the UsdMayaJobImportArgs structure.
• Handle parsing the new user args to build the UsdMayaJobImportArgs.
• Expose the upAxis and axisAndUnitMethod data to Python.
• Add the new UI related to upAxis and axisAndUnitMethod in the import UI.
• Add documentation about how to add a new import or export option.

Implement import rotation

• Add axis conversion helper to the UsdMaya_ReadJob class.
• Determine the Maya up-axis and USD up-axis.
• Do nothing if they match.
• Do nothing if the user requested to leave things as-is.
• Otherwise either change the Maya prefs or rotate the root imported objects.
• When rotating objects, group them under a group and rotate the group.
• The ungroup them if the user did not want to keep the group.
• Add original up axis attribute to the root nodes.
• Report success or failure of axis conversion.

Cleanup import code by refactoring into smaller functions to avoid code duplication.

Add unit tests for all three methods.