Added default reference pressure node for simulations with no Dirichlet pressure BCs
Minor changes to python code
Introduced a linearisation of generic scalar field source terms in an attemt to stabilise simulations with coupled species.  The darcy quadraticrelperm test folder has been modified to test a source term that is proportional to the prognostic scalar, but so far (a) only a vanishingly small coefficient of proportionality will result in acceptable convergence, whether or not the source term is linearised, and (b) the linearised source term is marginally worse than an unlinearised one and can be unstable at the same coefficient (as in the present test).  It could be that the linearisation implementation has a bug in it.  To be investigated.