Modify and update magnet meshing workflow

[connoramoreno]

Makes the following changes in an effort to improve the magnet meshing workflow, as well as fix some oversights in a previous PR:

• Changes magnet meshing algorithm in Cubit to one that gives greater control over mesh parameters, adding additional meshing arguments
• Puts checks in magnet meshing workflow to determine whether magnets have already been imported into Cubit, and if not, to properly set volume IDs
• Modifies magnet meshing call in Stellarator class to incorporate additional meshing arguments
• Moves magnet and IVB STEP imports in Cubit to corresponding objects
• Updates magnet STEP import in Cubit to properly set volume IDs, reflecting recent changes in STEP import order
• Updates tests to check that volume IDs are properly set