Added Doxygen config file and logo for generated HTML.
Chanegd output directory of generated HTML to 'build' directory.
Added doxygen comments to point/boundary types.
Added Doxygen comments to HashStructure-based inex structures.
Added Doxygen comments to kd-tree and Multigrid Tree.
Replaced all references to Real with ELEM_TYPE in all index structures. Real only used in test cpp files.