Add DynamicCostFunctionToFunctor.

This adds a new wrapper class called DynamicCostFunctionToFunctor
that closes a gap in the current API: the existing
CostFunctionToFunctor can only be used with a SizedCostFunction, where
the number and sizes of all parameter vectors are known at compile-time.
The DynamicCostFunctionToFunctor allows you to wrap a generic
CostFunction into a templated functor which can then be used in a
DynamicAutoDiffCostFunction.

Also updates the existing CostFunctionToFunctor class to internally use
DynamicCostFunctionToFunctor.

Change-Id: I088adc3271c58d2519126c27037c3576965a36d6