Investigate InvIso() usage in mortar

[ebchin]

It seems the mortar method is using InvIso() to find xi, eta coordinates of a point projected to the common plane while giving the current (i.e. non-projected) coordinates of the element. Potentially the element should have projected coordinates as well.

• Verify the mortar integral should be evaluated on the common plane
• Verify the inputs of InvIso() and the non-linear equation it is solving. Potentially add some documentation to avoid future confusion.