Have both subscribe and unsubscribe in the .cc file. This resolves an issue recently...

Have both subscribe and unsubscribe in the .cc file. This resolves an issue recently reported on the mailing list of strange crashes where someone links with the debug library but forgets to set -DDEBUG, which leads to a mismatch between what is compiled into their application (coming from the header file) and what they link with (coming from the library) and that leads to a completely unhelpful error message. The resulting code will be slower, but subscribing and unsubscribing is fortunately not something we do all the time.

git-svn-id: https://svn.dealii.org/trunk@32039 0785d39b-7218-0410-832d-ea1e28bc413d