Structure:
_magnetostaticField.py: Base class
  - {Stray,Oersted}Field derive from it
_detailsMagnetostaticField.py: static class variables for BEM stuff
  - K operator
  - BEM mesh
  - bemMesh <-> ngMesh mapping

{stray,oersted}Field.py:
  - Derive from _magnetostaticField.py
_details{Stray,Oersted}Field.py: static class variable for FEM stuff
  - forms, preconditioners, solver