#19 deriv of 2 variables in WCMDerivativeBean

[Tom Downey]

WCMExpressionInputBean and WCMExpressionFunctionBean
both can be functions of two variables (as well as of
one variable). WCMDerivativeBean can currently only
take the derivative of a function of one variable.

Is there a reason to have it support derivatives of two
variables? Or, just let this case be a hand-coded case?