Introduced an option for linearising generic scalar field sources.  This can stabilise simulations with coupled species.  Tests to follow.  Note that the present commit was put together from chunks of the diff with ba021105fe0b36b677e2e2cc669efdcccd70369b.  I tried cherry-picking the latter commit, but unfortunately it wasn't atomic enough.  Also I had a problem with trang (the RNG converter), so darcy_impes_options.rnc isn't matching darcy_impes_options.rng yet.
Changed darcy_impes_options.rng to allow PressureMesh to be chosen for generic scalar source terms (and gradients).  Again, darcy_impes_options.rnc needs updating.