WIP: support for multiple independent systems/hierarchies in a bundle

[kecnry]

import phoebe

b = phoebe.default_binary(force_build=True)
b.add_system('other_system')

TODO:

• system checks to take system as input
• compute checks to obey system@compute, and for ebv checks to iterate over all systems in all computes
• all backends need to obey system@compute, including phoebe looping over multiple selections
• ability to inject addl default_binary into new system, without having to build manually? Ability to copy one system (and its components) to a new system?
• only allow components tagged with a system in the system