Compute lambda from an eirpr object using the Ross-Macdonald model

This function computes `m` and `lambda` for the output of one of `xde_scaling_eir` or `xde_scaling_Z`. The outputs are attached to eirpr

xde_scaling_lambda(model)

Arguments

model

a list that defines an `ramp.xde` model (*e.g.*, generated by `xde_setup()`)