Metric concepts

 - new MetricSpace DigitalMEtricSpace concepts 
 - models have operator()(p,q) to get the distance between p and q (maybe as double for l_p metrics)
 - models have method rawDistance(p,q) with exact/raw representation of the distance (distance power p for l_p metrics)
 - new adapter to adapt any euclidean metric (values on R) to digital metric (values on Z) with std::ceil
 - concepts have been moved to concepts::
In the resulting doc, the symmetry item appears on the same line of identity