is_param — Check if a parameter is present in a parameter list
[res,err] = is_param(list_name,param_name)
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 add in the list of parameters.
the result: %T is the parameter is present, %F otherwise.
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).
This function checks if a parameter is present in a parameter list. If the err output parameter is not present, when an error occurs, a message is printed in the console.