Rename "step_type" (and variations) to "step_size"

[dtex]

@rwaldron pointed out that using step_type is confusing. If this property were intended to only differentiate between whole, half and micro stepping it would make sense, but we've engineered this so that we can support 1/4 step, 1/8, 1/16, etc if/when they become available so step_size makes more sense.

I don't believe this will be a breaking change and I'll get a PR in the next day or two.