Code review comment for lp:~jobh/dolfin/symmetric-assemble

I've fixed (or at least improved) the style issues, as requested.

I realise that the private-implementation (pImpl) style is not used much in dolfin, but I think it saves a lot of clutter in this case. Besides, the semi-public nature of methods like f.x. Assembler::assemble_exterior_facets forces a particular implementation and loop ordering, which complicates matters if the different implementations are to be merged.