Feature: Add support for spirals with non-integer turns

[dzimmanck]

When creating a winding that spans multiple layers, it is often useful to be able to generate spirals which have a non-integer number of turns. For example, if I wanted to create a 5 turn coil over 2 layers, then I would want to have 2.5 turns on each layer. Even in designs that technically have an integer # of turns, it is necessary to over-rotate each layer in order to create some overlap between spirals on different layers for placing via arrays to connect layers.