Change NumericDiffCostFunction to accept variadic functors.

The interface for NumericDiffCostFunction and AutoDiffCostFunction
are not comparable. They both accept variadic functors.

The change is backward compatible, as it still supports numeric
differentiation of CostFunction objects.

Some refactoring of documentation and code in auto_diff_cost_function
and its relatives was also done to make things consistent.

Change-Id: Ib5f230a1d4a85738eb187803b9c1cd7166bb3b92
7 files changed