Unitful Paths should always use preferred units

[gpeairs]

For convenience, default constructors like Path() and CoordinateSystem(name) use typeof(1.0UPREFERRED) as the coordinate type. However, if you have Path(0mm, 0mm), the coordinate type will be typeof(1.0mm). I can see very little reason to ever want that, and it tends to cause issues (not necessarily bugs but definitely bad user experience) related to conversions or assumptions about container eltypes. So that case should use the preferred unit as the coordinate type instead. If you want a different unit you have to specify it explicitly.