StMoMo2iMoMo: Transform a mortality rate model to an improvement rate model
In amvillegas/iMoMo: Mortality Improvement Rate Modelling

Transforms a StMoMo object representing a mortality rate model into an iMoMo object representing an improvement rate model

StMoMo2iMoMo(
  model,
  type = c("indirect", "direct"),
  constFun = function(ax, bx, kt, b0x, gc, wxt, ages) list(ax = ax, bx = bx, kt = kt,
    b0x = b0x, gc = gc)
)

`model`	A StMoMo object representing the mortality rate model to be transformed into an improvement rate model. This StMoMo model must include an static age function α_x and must use a `"log"` link.
`type`	defines the type of estimation method to be used. `"indirect"` would estimate the equivalent mortality rate model and the transform the model into improvement rates. `"direct"` would estimate the model directly on the improvement rate model.
`constFun`	function defining the identifiability constraints of the model. It must be a function of the form `constFun <- function(ax, bx, kt, b0x, gc, wxt, ages)` taking a set of fitted model parameters and returning a list `list(ax = ax, bx = bx, kt = kt, b0x = b0x, gc = gc)` of the model parameters with the identifiability constraints applied. If omitted no identifiability constraints are applied to the model.