Add support for passing a scalar GenericFunction as default value to
a CompiledExpression.

   from dolfin import *
   mesh = UnitSquareMesh(2,2)
   V= FunctionSpace(mesh, "CG", 1)
   f = Function(V)
   e = Expression("2*f", f=f)
   assert(assemble(2*f*dx, mesh=mesh)==assemble(e*dx, mesh=mesh))
Fix for parallel runs in unit test
Change check in generated eval code from dolfin_asserts to if->error.
Add support for setting name and label to an Expression when constructed
Small change to autogenerated name. It now follows the same convention as Function and Constants
Small update in Expression.py test.
  -- PETSc 3.2 fails if update_ghost_values are called in serial
Merge branch 'johanhake/generic_functions_in_compiled_expressions'