Pinning linear solves

[landinjm]

I've added a user input parameter that specifies where on the mesh the solution is pinned to zero. You can technically do this with any field type, but it should mainly be used in adding constraints to linear/nonlinear solves.

This works in addition to any other boundary conditions.

To add a pinning point, add something like this in parameters.prm, where n is the name of the field you want pinned.

subsection Pinning point: n
    set x = 0.0
    set y = 0.0
end

Closes #182

[landinjm]

I've also deleted the getRigitBodyConstraints unit test as it is now obsolete.