Make the FreeCAD path configurable throughout the package

[arewellborn]

Is your feature request related to a problem? Please describe.
Currently the path pointing to the FreeCAD binary files is hard-coded to "C:/Program Files/FreeCAD 0.19/bin". I am not able to use my local version of FreeCAD because of this.

Describe the solution you'd like
A way to configure the path to the FreeCAD bin directory using an environment variable.

Describe alternatives you've considered
A way to pass the file path down to the various methods that use it, or a mutable "config" object in planar_magnetics.

Additional context
Methods that require the FreeCAD path are:

• geometry.py::Polygon.to_wire
• transformers.py::Transformer.to_kicad_footprint
• cores.py::Core.to_parts
• cores.py::Core.to_step

[arewellborn]

I've drafted up a mock-up of what I was thinking above. Let me know if you agree with the changes or approach.