Distance visitor based local estimator adapter

The main class of this PR implements a generic adapter to construct a local estimator on digital surfaces based on neighboring surfels at each surface element.
This adpater needs:
   - a metric to define the neighborhood (and a radius during init())
   - a kind of functor which is executed on each neighborhood.
 
At this point, this PR contains only the main classes and concept. A dummy example of local functor is given (we just count the number of surfels) to illustrate the process.

More functor (e.g. local polynomial fitting) will be given later.