Explain how to only evaluate J in OTF (no optimi)
Remove k as a parameter
2 Changes to get_distance_function:

1) Use a linear solver instead of Newton
2) The "domains" parameter is now expected to be a dolfin.Expression (before dolfin.MeshFunction).
   This avoids the unsafe conversion from MeshFunction -> Function which caused trouble in parallel.
Fix the domain_constraint test
Remove the eta0 parameter. Fix the gradient_friction test
Fix the remaining tests
Revert "2 Changes to get_distance_function:"

This reverts commit 9ea0a5f0a30914442020e8041affc11b13ad1aa5.
Revert "Fix the domain_constraint test"

This reverts commit 737ca50edbc8b4ffd340d84edd2ed9a8920a7288.