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

include/ceres/types.h

@@ -335,6 +335,11 @@ enum DimensionType {
   DYNAMIC = -1
 };
 
+enum NumericDiffMethod {
+  CENTRAL,
+  FORWARD
+};
+
 const char* LinearSolverTypeToString(LinearSolverType type);
 bool StringToLinearSolverType(string value, LinearSolverType* type);