Simplify logic in the PyDOLFIN expression class

Make Expression a sub class of cpp.Expression, ufl.Function. This class should only be used when subclassing in Python.

This makes sub classing in Python intuitive and users should not get any nasty surprises when initializing base classes.

We can consider adding functionalities for:

  def eval(value, x):
      values[0] = sin(x[0])

   e = Expression(eval, V=V)

Add CompiledExpression. A function that wrap the compiled expression stuff.