Bulk generation and generation by spacing

[cst0]

Thanks for sharing this project! It's been super handy. There were two features that I needed and implemented for myself, so I thought I'd share them in case there's any interest in merging.

The first feature is coil generation by coil spacing: I wanted my coils to be as tight as allowed by the manufacturer, I didn't really care about the number of windings that produced. So, I allowed the 'n_turns' parameter to be None as long as 'spacing' is not None. If 'spacing' is provided, the n_turns is calculated using that information.

The second feature is bulk generation: a little bit of refactoring allowed the core functionality of this library to be called from an external script, that external script iterates over a handful of parameters to produce a set of footprints.

Let me know if you're interested in this functionality, and/or if there's any changes you'd like me to make here!