Don't normalize A_3 vector

Comment added to explain why
Oops, stress_and_elasticity calculation not ready yet
Move det_a calculation earlier
This formulation is in convected curvilinear coordinates

Meaning that I'm an idiot. Namely, all the derivatives are w.r.t.
*master element coordinates*. *NOT* spatial coordinates. At this
point, both elastic_sheet and inflating_sheet are giving me reasonable
results.