Store user defined parameter in CompiledExpressions. Making the
following possible:

   from dolfin import *
   e = Expression("0.1*a+b", a=1.0, b=Constant(3.0))
   assert(e(0.0)==3.1)
   if "a" in e.user_parameters:
       e.user_parameters["a"] = 2.0
   assert(e(0.0)==3.2)
   params = dict(a=3.0, b=Constant(1.0))
   e.user_parameters.update(params)
   assert(e(0.0)==1.3)
Merge branch 'johanhake/named_expression_params'