Make FillJacobianBlock public in ConstraintSet

[mpowelson]

By allowing this to be externally callable, we are able to make costs and constraints that modify costs and constraints passed into them.

An example is the squared cost I made in order to address towr issue 72. The idea is that you can calculate any values you want as constraints and the pass them into this squared cost to convert them to into a squared cost. The issue was that I was only able to call the GetJacobian of the contained constraint which I would then have to decompose into the form that FillJacobianBlock needs. By exposing the constraint FillJacobianBlock, I can get only the pieces that the constraint I am modifying needs.

[awinkler]

Yes, that works, thanks for your help!