changing system hierarchy with flipped constraints fails

[kecnry]

changing system hierarchy (binary->triple, flipping roles of primary and secondary, etc) throws an error when any of the built-in constraints have been flipped.

[kecnry]

note that this is still a bug, but is probably not a common scenario. To reproduce:

import phoebe
b=phoebe.default_binary()
b.flip_constraint('mass@primary', solve_for='period')
# setting the hierarchy still does work at this point, but flipping the constraint back causes issues
b.flip_constraint('period@binary', solve_for='mass')
b.set_hierarchy('orbit:binary(star:secondary, star:primary)')