add .gitattributes

we ignore the testsuite and some other files when creating a tarball
with git archive. This is also used by github for 'releases'.
improve SolutionTransfer exception

parallel::distributed::SolutionTransfer only works with parallel::distributed::Triangulation. This is now checked in the constructor.
fix doxygen formatting

reformat a couple of places in the doxygen comments manually (broken
@image lines etc.). This is mostly to make wrapcomments.py happy.
update wrapcomments.py

now handle @image tags correctly
remove stray comment
put @ref in separate line
force @ref in separate line
fix more files
fix up more files
fix warpcomments.py
format everything
more fixup
fixup files
Now also cached variables have CPACK prefix.
Merge pull request #380 from tamiko/dealii-8.2

cherry-pick commits to the release branch