Helper to define list of parameters to control the PLNmixture fit. All arguments have defaults.

```
PLNmixture_param(
backend = "nlopt",
trace = 1,
covariance = "spherical",
init_cl = "kmeans",
smoothing = "both",
config_optim = list(),
config_post = list(),
inception = NULL
)
```

## Arguments

- backend
optimization back used, either "nlopt" or "torch". Default is "nlopt"

- trace
a integer for verbosity.

- covariance
character setting the model for the covariance matrices of the mixture components. Either "full", "diagonal" or "spherical". Default is "spherical".

- init_cl
The initial clustering to apply. Either, 'kmeans', CAH' or a user defined clustering given as a list of clusterings, the size of which is equal to the number of clusters considered. Default is 'kmeans'.

- smoothing
The smoothing to apply. Either, 'none', forward', 'backward' or 'both'. Default is 'both'.

- config_optim
a list for controlling the optimizer (either "nlopt" or "torch" backend). See details

- config_post
a list for controlling the post-treatments (optional bootstrap, jackknife, R2, etc.).

- inception
Set up the parameters initialization: by default, the model is initialized with a multivariate linear model applied on
log-transformed data, and with the same formula as the one provided by the user. However, the user can provide a PLNfit (typically obtained from a previous fit),
which sometimes speeds up the inference.

## Value

list of parameters configuring the fit.

## Details

See `PLN_param()`

for a full description of the generic optimization parameters. PLNmixture_param() also has additional parameters controlling the optimization due the inner-outer loop structure of the optimizer:

"ftol_out" outer solver stops when an optimization step changes the objective function by less than xtol multiplied by the absolute value of the parameter. Default is 1e-6

"maxit_out" outer solver stops when the number of iteration exceeds maxit_out. Default is 50

"it_smoothing" number of the iterations of the smoothing procedure. Default is 1.