Implement a scheme to free unused memory for dVector and dSMatrix. The memory is...

Implement a scheme to free unused memory for dVector and dSMatrix. The memory is not freed (as before) if the object is made smaller, but is now freed altogether if the object is assigned the size zero. This functionality is needed if you really want to free all memory, e.g. to let an object sleep for a while but don't want to use those unhandy pointers for half a dozen vectors and matrices. -- Hopefully the new scheme doesn't break too many other things; the previous state of the library is a bit hacked together (my fault also, I admit...).

git-svn-id: https://svn.dealii.org/trunk@373 0785d39b-7218-0410-832d-ea1e28bc413d