[RST-4390] Allow variables to be held constant during optimization

[svwilliams]

Ceres Solver allows you to mark variables as constant. Ceres will not alter the values of those variables during the optimization. In fact, it actually removes those variables from the linear optimization problem when solving for the best variable values. Add support for constant variables to fuse as follows:

• Provide an accessor method in the Variable base class for checking if a variable is held constant. Default to return false.
• Update the graph implementation to check the new variable property during insert and add them to the set of constant variables.
• Update the marginalization code to check for constant variables and set their jacobians to zero. This is the rough equivalent of removing them from the linear problem entirely.
• Add a unit test to verify the marginalization code works -- verify the remaining variables optimize to the same value before and after a constant variable is marginalized out.
• Add a "fixed landmark" variable that uses the new "hold constant" feature

[efernandez]

@svwilliams Just curious, I wonder if this is the only difference wrt Point3DLandmark, why Point3DFixedLandmark wasn't simply implemented as:

class Point3DFixedLandmark : public Point3DLandmark
{
public:
  FUSE_VARIABLE_DEFINITIONS(Point3DFixedLandmark);

  bool holdConstant() const
  {
    return true;
  }
};

and the corresponding code for the serialization and pluginlib in the cpp file. 🤔

[svwilliams]

Honestly, I think just an accident of history.

[svwilliams]

#259