get_optim_fit_triphasic: Fit triphasic model to data using optim
In ushr: Understanding Suppression of HIV

This function uses optim to fit the triphasic model to data from a given subject

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get_optim_fit_triphasic(initial_params, param_names, free_param_index,
  data, forward_param_transform_fn = forward_param_transform_fn,
  inv_param_transform_fn = inv_param_transform_fn, searchmethod)

`initial_params`	named vector of the initial parameter guess.
`param_names`	names of parameter vector.
`free_param_index`	logical vector indicating whether the parameters A, delta, A_b, delta_b, B, gamma are to be recovered. This should be c(TRUE, TRUE, TRUE, TRUE, TRUE, TRUE) for the triphasic model.
`data`	dataframe with columns for the subject's viral load measurements ('vl'), and timing of sampling ('time')
`forward_param_transform_fn`	list of transformation functions to be used when fitting the model in optim. Defaults to log transformations for all parameters (to allow unconstrained optimization).
`inv_param_transform_fn`	list of transformation functions to be used when back-transforming the transformed parameters. Should be the inverse of the forward transformation functions. Defaults to exponential.
`searchmethod`	optimization algorithm to be used in optim. Defaults to Nelder-Mead.