The changes separate the decisions of the levelsets from the Heaviside
function, s.t. the behaviour does not change if the Heaviside function
is replaced with a different step function.

Replace displacement controlled top with traction and add
traction to left and right boundaries to avoid induced rotation.

Used in LIE for the stress/strain output, where the fracture
(lower order) elements should not contribute to the extrapolation.

This helps with the Neumann type boundary conditions on boundaries
crossed by fractures. See LIE/M two-fractures push-pull test.

The access might fail silently otherwise.

CHA - constant hydraulic aperture.