Changed darcy_impes_options schema to allow PressureMesh to be chosen for generic scalar source terms (and gradients).
Modified the relative permeability relations such that the relperm does not go below zero.  Untested.
Added do_not_dilute option.
Debugged generic fields in dual permeability mode
The generic source linearisation feature is confirmed to be working and is now tested in the quadraticrelperm folder.  However, the more recent non-negative effective saturation feature is incompatible with MMS tests (because the relperm relation is undifferentiable), so it has been made optional.