initialized variable to avoid compiler warning
updated implementation given the fact that some parts of the code have been moved to algorithms/detail/distance/closest_distance_rtree.hpp
renamed file
polished implementation of closest distance that uses the R-Tree query iterator;
updated benchmarking programs to work again with the latest code changes;
the comparable to regular conversion is not coordinate-system dependent, and the existing implementation is now explicitly defined in the Cartesian CS.
removed files in multi/algorithms/detail/distance dir; code has been moved to same files in algorithms/detail/distance dir;
reverted previous changes; no need for them any more due to the directory/file layout changes in distances;
reverted changes that were made in order to make R-Tree work within distances;
changes are no longer needed due to the new way the file/directory structure of BG is designed;
implementation of the distance algorithm using the new file/directory structure:
dispatch is at algorithms/dipatch/distance.hpp
all details (including multi-geometries) are in the files of the algorithms/detail/distance directory
multi/algorithms/distance.hpp is now empty of code (it is there as a place-holder in order to avoid unnecessary file inclusions from other versions of BG);
unit tests have been modified appropriately to include algorithms/dispatch/distance.hpp as well;
the unit test multi_distance.cpp fails due to a clash with R-Trees (failing code has been commented);
unified copyright headers
polished file includes;