SensorModel factory for external models

Part of #1506 (closed)

This item has a low priority for now, but it will be a real asset in the future.

The idea is to improve the SensorModel factory mechanism to load external sensor models. They should be used in association with the metadata entry MDGeom::SensorModel. The model is stored as a boost::any, the SensorModel will be in charge of its usage and serialization (so that GDAL can read/write the model to its metadata).

The SensorModel should derive from otb::GenericRSTransform (after its refactoring).

I don't think we need to include the optimization functions in the SensorModel (AddTiePoint, Optimize), they can be handled separately. The optimization functions are not top priority in this refactoring.

On the packaging side, we can:

• use a plugin mechanism to detect external sensor models in lib/otb/plugins
• use the variable ITK_AUTOLOAD_PATH to load the plugins

The tests related to Spot-5 were deactivated while implementing RPC models, because it is not based on RPC but in sensor physical characteristics. When the new SensorModel factory is implemented, those tests need to be reactivated:

• prTvSensorModel_spot5-1