Internal function to update one aspect of prms_solve or solver.
set_one_solver_setting(drift_dm_obj, name_prm_solve, value_prm_solve)the updated un-evaluated (!) drift_dm_obj object
an object of type drift_dm
which aspect to address? ("sigma", "t_max", "dx", "dt", "solver")
either a single numeric or character string
Ensures that the supplied values are reasonable and that nx and nt are
updated. The functions prms_solve<- and
solver<- pass their arguments forward to this function.