FactorAutodiff reimplementation: from pointers to mutable

Just an implementation detail. Since ceres requires evaluate() to be const. In FactorAutodiff our implementation of the differentiation wanted to reuse the Jets so the attributes were pointers to them. In this way, the Jets could be changed in a const function.

Anyway, I just discovered mutable keyword that means exactly this if applied to an attribute: it can be changed in a const method. So instead of this ugly workaround, code should be changed to mutable Jets.