Add line-horizontality constraint

[tomcur]

No description provided.

[tomcur]

There is some subtlety between this and the recursively assembling decomposition in #94, as with that method a lot of rigid transformations take place, which includes rotation, whereas we assume here that we're constraining against the global coordinate system.

There may be a way to use that decomposition method such that we can still assume we're naturally referencing the global coordinate system for these types of constraints, but I haven't fully thought it through yet. The short version is that any constraint referencing the global coordinate system (like this constraint here, or any constraint between free and anchored elements impacting positioning), is against a dummy element in the graph analysis representing the global coordinate system (with 0 DOF). The first cluster to include that dummy element will have its local coordinate system selected from that point forward as the target coordinate system.