[doc][doxygen] Remove doxygen errors.

doc/doxygen/footer.html

doc/doxygen/groups/models.md

doc/doxygen/groups/multidomain.md

doc/doxygen/header.html

dumux/common/balanceequationopts.hh

dumux/common/boundaryflag.hh

dumux/common/boundarytypes.hh

dumux/common/cubicspline.hh

dumux/common/cubicsplinehermitebasis.hh

dumux/common/defaultmappertraits.hh

dumux/common/defaultusagemessage.hh

dumux/common/deprecated.hh

dumux/common/dimensionlessnumbers.hh

dumux/common/doubleexpintegrationconstants.hh

dumux/common/doubleexpintegrator.hh

dumux/common/dumuxmessage.hh

dumux/common/entitymap.hh

dumux/common/enumerate.hh

dumux/common/exceptions.hh

dumux/common/fixedlengthspline_.hh