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 =
I agree it is a good idea. How will we go about deciding when it finally should be removed? And should we put that in the deprecation message if we can call it now?