Add constructor for OptionalJacobian

[gchenfc]

The fixed-size version of OptionalJacobian doesn't have a constructor for pointers to dynamic matrices. This makes it cumbersome to deal with e.g.:

double myFunc(double x, gtsam::OptionalJacobian<6, 6> H = boost::none) { ... }

class MyFactor : public gtsam::NoiseModelFactor {

  ...

  Vector unwhitenedError(const Values& x,
        boost::optional<std::vector<Matrix>&> H = boost::none) const override {
    double y = x.at<double>(keys_[0]);
    // myFunc(y, H ? &(*H)[0] : nullptr); //< this doesn't compile

    return H ? myFunc(y, (*H)[0])
             : myFunc(y); //< this does compile but is not as nice
  }
};

Frank recommends using the former "modern" syntax but this isn't possible without adding the constructor overload:

OptionalJacobian(Eigen::MatrixXd* dynamic) {...}

For future readers who are less familiar, ternary operator must have same-types for the middle and last parameters, and since the last parameter is a null then the middle parameter (and the whole output-type) has to be a pointer type.

No breaking changes and nothing removed, just added a new constructor overload.

[varunagrawal]

I'm wondering if we can use boost::none instead of nullptr.

[varunagrawal]

LGTM

[gchenfc]

Ya, in this case I used nullptr to stay consistent with the rest of the file, but I guess I usually use nullptr for brevity - do you think there's enough advantage that I should use boost::none in the future?