LinearFactorContainer rekey()

[svdt]

Hi,
I am getting seg faults or ValuesKeyDoesNotExist errors when using rekey on the LinearFactorContainer.

Description

As the linearization point is optional and there are checks for linearizationPoint_ in some methods, the rekey method is missing these checks.
https://github.com/borglab/gtsam/blob/develop/gtsam/nonlinear/LinearContainerFactor.cpp#L180

Steps to reproduce

1. create LinearContainerFactor with GaussianFactor but without a linearization point
2. rekey the LinearContainerFactor

#include <gtsam/linear/HessianFactor.h>
#include <gtsam/nonlinear/LinearContainerFactor.h>

int main(int argc, char *argv[]) {

    double mu = 1e10;
    size_t dim = 3;
    gtsam::Key key = 0;

    gtsam::HessianFactor H(key, mu * gtsam::Matrix::Identity(dim, dim), gtsam::Vector::Zero(dim), 0.0);
    gtsam::LinearContainerFactor factor(H);

    gtsam::Key new_key = 1;
    std::map<gtsam::Key, gtsam::Key> rekey_mapping = {{key, new_key}};
    factor.rekey(rekey_mapping);

    gtsam::KeyVector rekey_vector = {new_key};
    factor.rekey(rekey_vector);

    return 0;
}

Expected behavior

Remap the keys of the GaussianFactor and only modify linearizationPoint_ if it has a value.

Environment

C++, Ubuntu 20.04, gtsam develop branch commit: 164c354