fit.iMoMoI: Fit an Improvement Rate Model indirectly
In amvillegas/iMoMo: Mortality Improvement Rate Modelling

Fit an Improvement Rate Model indirectly via fiting the equivalent mortality rate model and then trasforming the output to improvement rates

## S3 method for class 'iMoMoI'
fit(
  object,
  data = NULL,
  Dxt = NULL,
  Ext = NULL,
  ages = NULL,
  years = NULL,
  ages.fit = NULL,
  years.fit = NULL,
  wxt = NULL,
  start.ax = NULL,
  start.bx = NULL,
  start.kt = NULL,
  start.b0x = NULL,
  start.gc = NULL,
  verbose = TRUE,
  ...
)

`object`	an object of class `"iMoMoI"` defining the improvement rate model.
`data`	an optional object of type StMoMoData containing information on deaths and exposures to be used for fitting the model. This is typically created with function `StMoMoData`. If this is not provided then the fitting data is taken from arguments, `Dxt`, `Ext`, `ages`, `years`.
`Dxt`	optional matrix of deaths data.
`Ext`	optional matrix of observed exposures of the same dimension of `Dxt`.
`ages`	optional vector of ages corresponding to rows of `Dxt` and `Ext`.
`years`	optional vector of years corresponding to rows of `Dxt` and `Ext`.
`ages.fit`	optional vector of ages to include in the fit. Must be a subset of `ages`.
`years.fit`	optional vector of years to include in the fit. Must be a subset of `years`.
`wxt`	optional matrix of 0-1 weights to be used in the fitting process. This can be used, for instance, to zero weight some cohorts in the data. See `genWeightMat` which is a helper function for defining weighting matrices.
`start.ax`	optional vector with starting values for α_x.
`start.bx`	optional matrix with starting values for β_x^{(i)}.
`start.kt`	optional matrix with starting values for κ_t^{(i)}.
`start.b0x`	optional vector with starting values for β_x^{(0)}.
`start.gc`	optional vector with starting values for γ_c.
`verbose`	a logical value. If `TRUE` progress indicators are printed as the model is fitted. Set `verbose = FALSE` to silent the fitting and avoid progress messages.
`...`	arguments to be passed to or from other methods. This can be used to control the fitting parameters of `gnm`. See `gnm`.