What's the difference between `orbit.fieldtracker` and `orbit.core.fieldtracker`?

[woodtp]

I'm tracking down compilation errors for #100. While building the docs, sphinx is unable to import from orbit.fieldtracker.

For example,

>>> import orbit.fieldtracker
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import orbit.fieldtracker
  File "/<redacted>/Projects/pyorbit3/py/orbit/fieldtracker/__init__.py", line 7, in <module>
    from . import FieldTracker
ImportError: cannot import name 'FieldTracker' from partially initialized module 'orbit.fieldtracker' (most likely due to a circular import) (/<redacted>/Projects/pyorbit3/py/orbit/fieldtracker/__init__.py)
>>>

Looks like FieldTracker should be imported from orbit.core.fieldtracker instead. What is the difference between orbit.fieldtracker and orbit.core.fieldtracker? Were these unified at one point?
py/orbit/fieldtracker/init.py probably needs to be modified to look something like

from .TeapotFieldLatticeModifications import addTeapotFieldTrackerNode
from .TeapotFieldTrackerNode import TeapotFieldTrackerNode

__all__ = []
__all__.append("addTeapotFieldTrackerNode")
__all__.append("TeapotFieldTrackerNode")

[woodtp]

Similarly, this line

PyORBIT3/py/orbit/fieldtracker/TeapotFieldTrackerNode.py

Line 18 in 7c67f1c

from fieldtracker import FieldTracker

should be from orbit.core.fieldtracker import FieldTracker.

And this one

PyORBIT3/py/orbit/fieldtracker/TeapotFieldLatticeModifications.py

Line 11 in 7c67f1c

from orbit.FieldTracker import TeapotFieldTrackerNode

should be from .TeapotFieldTrackerNode import TeapotFieldTrackerNode.