Distance function in 1_4

[philippberens]

The way the distance function in 1_4 under 1_4_3 is implemented is incorrect, I believe. The reason is that the time basis is identical for all mortar simulations, but not the distance. Therefore, the distance function does not actually measure how close to the generated simulations are to the reference simulation. Also, I would restrict measuring distances for positive values of height, as negative values are meaningless.