Demand default value for expression variables

When constructing a compiled Python expression with variables, we should demand a passed default value. This will hide any potential misspellings of builtin DOLFIN variables as DOLFIN_EPS. We can pass them as **kwargs:

  f = Expression(a*sin(x[0]), a=1.0)

Probably need to hide the accepted kwargs (element, aso) as these should come behind the default value declaration.