Use `ufl` instead of `dolfin` in definition of variational forms

[uvilla]

dolfinx is restructuring the way dolfin packages and functions are imported.

A small step towards supporting dolfinx (or maybe even firedrake is to use directly functions from the ufl package when defining weak forms.

This includes:

• Basic ufl nonlinear functions: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#basic-nonlinear-functions

• Tensor algebra operators: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#tensor-algebra-operators

• Differential operators: ufl.grad, ufl.div, ufl.curl, ufl.rot (Also replace all nabla_grad with grad.)

• Discontinuous Galerkin operators: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#dg-operators

• Conditional operators: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#conditional-operators

• ufl.rhs and ufl.lhs: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#linear-and-bilinear-parts-of-a-form

• ufl.derivative: https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#automatic-functional-differentiation

• ufl.split: (split a Mixed Functions into its components)

• integrators: ufl.dx, ufl.ds, ufl.dS, ufl.dP

[uvilla]

Closed by PR #46