add_param — Add a parameter to a list of parameters
[param_list,err] = add_param(list_name,param_name,param_value)
the list of parameters. This list must have been initialize by a call to init_param.
a string. The name of the parameter to be added in the list of parameters.
the value associated to the parameter param_name. This parameter is optional. You can set the value of this parameter via a call to set_param.
the updated list of parameters.
an error flag which is set to %T if list_name is not of type plist (this list hasn't been initialized by a call to init_param).