if target is null, we'll try to minimize the risk metric
portfolio_risk_objective(name, target = NULL, arguments = NULL,
multiplier = 1, enabled = TRUE, ...)
object of class 'portfolio_risk_objective'
name of the objective, should correspond to a function, though we will try to make allowances
univariate target for the objective
default arguments to be passed to an objective function when executed
multiplier to apply to the objective, usually 1 or -1
TRUE/FALSE
any other passthru parameters
Brian G. Peterson