Remove @nonlinear_form functionality and modify example

[kinnala]

I tried extending @nonlinear_form to vectorial elements and solved St. Venant-Kirchhoff hyperelastic problem. The resulting code, evaluation of the Jacobian using autograd, turned out to be so slow that it was basically unusable. Unless we have a good alternative to autograd, I think this functionality is unnecessary: the user can always compute Jacobian by hand and use @bilinear_form and @linear_form which results in a faster implementation.

Since I think we should focus on the core functionality of the library, I propose that we remove @nonlinear_form altogether and modify ex10.py so that the Jacobian is computed symbolically.

[kinnala]

This was done in 75afe58.