update margin inversion to allow for greater than or less than symbols

When trying to change some sideMargin constraints, I learned that there was no handling for constraints with a relationship of greater than or less than. closes https://github.com/orta/ORStackView/issues/14.

I think in the future it would be best to take this a step further and instead of constraining the width and aligning center, actually align the leading and trailing edges using the sideMargin value provided. Right now the left and right margins will each be 1/2 of the sideMargin provided, which is a bit misleading.