plan class
use plans in constraint system
fix errors
rename constraints
Merge branch 'master' of github.com:knub/skypyblue into using-plans

Conflicts:
	src/skypyblue/core/constraint_system.py
	src/skypyblue/models/constraint.py
	tests/mvine_tests.py
Merge branch 'master' of github.com:knub/skypyblue into using-plans

Conflicts:
	src/skypyblue/core/constraint_system.py
Merge branch 'master' of github.com:knub/skypyblue into using-plans

Conflicts:
	examples/midpoint_example.py
remove old tests
[ci skip] integrated plan, but tests failing
[ci skip] fixed x is not in list error
[ci skip]
root constraints are just constraints
[ci skip]
Merge branch 'using-plans' of github.com:knub/skypyblue into using-plans
[ci skip] list comprehesion instead of for loops
[ci skip]
test chain of constraints
Merge branch 'using-plans' of github.com:knub/skypyblue into using-plans
fixed: bad and good constraints are now lists
fixed tests for python3
changed classes, so they inherit all from object
removed unnecessary comment
Merge branch 'using-plans' of github.com:knub/skypyblue