get_param — Get the value of a parameter in a parameter list
[res,err] = get_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 value of the parameter. If the parameter doesn't exist, res = [].
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).