Opt.addPrecondEqualityConstraint

Add pre-conditioned equality constraint

Refer to http://ab-initio.mit.edu/wiki/index.php/NLopt_Reference#Preconditioning_with_approximate_Hessians for information on preconditioning.

Equivalent in C API: nlopt_add_precond_equality_constraint

Params: (TODO)

fc = function pointer representing constraint, must be extern(C)

pre = function pointer representing preconditioner function, must be 
		     extern(C)

fc_data = additional data to pass to functions (optional)

tol = tolerance for constraint (default set to defaultTol.equality)