EstimatorCache

New model of CSurfelLocalEstimator that acts as a cache of an existing estimator. 

More precisely, when "init()" method is called, all estimated values are computed and stored in a container. The eval() method of the cache is just an access to the associative container Surfel<->Quantity.

Example Use-case: estimate once the normal vector field and use it efficiently several times later. 


(spherefitting will come in a separatePR)
why calling it "Gaussian"Cache ?