added method that lets caller specify relation between spaced views

original method now calls the new one with NSLayoutRelationGreaterThanOrEqual as before

@ndonald2 I needed this functionality to have some labels display correctly when not pinning their widths; otherwise they'd shrink due to the >= relation of the spacing. This way I can set it to =
Don't worry about the separate category for now, and I wouldn't even worry about the `@warning`. This is going away soon.