Corrected Doxygen warnings for Doxygen 1.8.5

After running make doc I obtain the following warnings on my system (Doxygen 1.8.5).
This pull request turns them off.

Warnings
====
Warning: Tag `SYMBOL_CACHE_SIZE' at line 289 of file Doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `SHOW_DIRECTORIES' at line 473 of file Doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `HTML_ALIGN_MEMBERS' at line 799 of file Doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: doxygen no longer ships with the FreeSans font.
You may want to clear or change DOT_FONTNAME.
Otherwise you run the risk that the wrong font is being used for dot generated graphs.
/home/thillux/Desktop/osmium/include/osmium/utils/filter_and_accumulate.hpp:33: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/utils/filter_and_accumulate.hpp:34: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/utils/filter_and_accumulate.hpp:35: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/utils/filter_and_accumulate.hpp:36: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/tags/to_string.hpp:37: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/tags/to_string.hpp:38: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/tags/to_string.hpp:39: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/tags/to_string.hpp:40: warning: Found unknown command `\parameter'
/home/thillux/Desktop/osmium/include/osmium/tags/to_string.hpp:41: warning: Found unknown command `\parameter'