Add function to compute gaps for Mortar

[ebchin]

It would be convenient to have a way to compute gaps without updating forces and Jacobian contributions in the mortar method. See this code in Serac. There, when we are using a penalty enforcement of the constraint we have to call update() twice for the code to work properly: once to update the gaps for computing the updated pressures, then again to compute the forces and Jacobian with the updated pressures.

[srwopschall]

Yeah right now you'd have to modify the implicit evaluation option to compute gaps only and then call update(). I don't think the gap, residual, jacobian calculations in the mortar physics routine are guarded correctly, but that is an easy fix. We could easily write an API function that computes the gaps for a given coupling scheme and currently registered mesh data, but if you wanted to update the geometry of the overlaps you may as well call update. If that feels awkward, maybe we have an API routine that actually wraps update(), but to the user feels like they are getting what they are asking for? Let's discuss more.