[algorithms][distance] change calls to bg::within by the more stable
bg::covered_by; rename template parameter in partial specialization of
point_to_multigeometry to reflect the actual geometry for which it is
called;
[algorithms][distance] replace the default value for the boolean template
parameter CheckCoveredBy to the correct one
[algorithms][distance] remove unused variables
[algorithms][closest_feature] for each intermediate distance result
add test against zero, and return if needed
[algorithms][distance] remove the UseRTreeOfSegments template parameter
and the specialization when the value of this parameter was false
[algorithms][distance] remove the geometry_to_geometry_rtree class
(not used anymore); rename "cf" to "closest_features" (more descriptive);
fix header includes;
[algorithms][distance] rename the file "geometry_to_geometry_rtree.hpp"
to "range_to_geometry.hpp"