Enhancements to analytic fields in magneticfieldclasses.py

[smoniewski]

While adding dofs to CircularCoil in #377, a couple points were raised. I thought they were out of the scope of the pull request, so I am repeating them here.

1. Updated CircularCoil class to alllow for dofs #377 (comment) > what about the other magnetic fields in this file (ToroidalField, PoloidalField, DipoleField, etc)? they don't have this added support. Not saying that we need to, but just something that I noticed.
2. Updated CircularCoil class to alllow for dofs #377 (comment) > this field is tied to a specific coil, but that coil is not a geometry object. So this coil will not work for any of the regular curve objectives. Perhaps we should have a CircularCoil curve object that gets its field from CircularCoilField.