Add support for up to 10 parameters in autodiff.

Supporting only 6 parameters in autodiff was enough for most
cases, but 6 was not always sufficient. This extends the
current implementation to work with up to 10 parameters.
This also increases the number of parameters supported in
SizedCostFunction to 10.

Change-Id: Ic783602f93e6ddf4af24fa34eff37c0a4b775dc1