Write section on implementation of new gravity models

[MiguelAvillez]

Modifications that were necessary to implement the polyhedron gravity model are listed below. Same changes would be necessary for any other new gravity model.

• createGravityField.h/cpp: Added PolyhedronGravityFieldSettings class, edited createGravityFieldModel function, added new polyhedronGravitySettings functions
• polyhedronGravityField.h/cpp: created files. Added PolyhedronGravityField and PolyhedronGravityCache classes; other functions
• polyhedronGravityModel.h/cpp: created files
• Third body gravity:
  • thirdBodyPerturbation.h: added ThirdBodyPolyhedronGravitationalAccelerationModel typedef
  • tudatTypeTraits.h: added polyhedron to gravitation type traits
• createAccelerationModels.h/cpp:
  • New functions: createPolyhedronGravityAcceleration, createThirdBodyPolyhedronGravityAccelerationModel
  • Modified functions: createDirectGravitationalAcceleration, createThirdBodyGravitationalAcceleration,
    createGravitationalAccelerationModel, createAccelerationModel
• accelerationModelTypes.h/cpp:
  • added polyhedron to enum (2 times)
  • Modified functions: getAccelerationModelName, getAccelerationModelType, isAccelerationDirectGravitational,
    isAccelerationFromThirdBody, getAssociatedThirdBodyAcceleration
• accelerationSettings.h: new settings function
• createEnvironmentUpdater.cpp:
  • Modified functions: createTranslationalEquationsOfMotionEnvironmentUpdaterSettings
• Dependent variables: propagationOutput.h
  • Add enum to getGravitationalAccelerationForDependentVariables
  • getDoubleDependentVariableFunction: add gravity model to gravity_field_potential_dependent_variable and gravity_field_laplacian_of_potential_dependent_variable switch/case
• Variational equations for polyhedron gravity
  • polyhedronAccelerationPartials.h/cpp: created files
  • createAccelerationPartials.h: edited createAnalyticalAccelerationPartial function
  • thirdBodyGravityPartial.h: edited getAccelerationTypeOfThirdBodyGravity, getCentralBodyNameFromThirdBodyAccelerationPartial
  • tudatTypeTraits.h: added polyhedron to acceleration_partials type traits