To enable natural convection, adjusted the darcy schema so that density could be made into a diagnostic field.  This is a quick and dirty implementation so as not to break users' options files; changes to the lower-level code may be needed in due course.  For MMS-based tests, split the generated solution dictionary in two; one for pure python code (numbers and lambdas) to be reused in sympy-decoupled code; the other for text substitution in templates.