This function can be used in projectToSteady() to decide when sufficient
convergence to steady state has been achieved.
distanceMaxRelRDI(params, current, previous)The largest absolute relative change in rdi:
max(abs((current_rdi - previous_rdi) / previous_rdi))
MizerParams
A named list with entries n, n_pp and n_other
describing the current state
A named list with entries n, n_pp and n_other
describing the previous state
Other distance functions: 
distanceSSLogN()