Circumvent a bug in Trilinos 11.10

If we avoid calling GlobalAssemble when copying from an Epetra_FECrsMatrix that uses a
non-local matrix (enabled for Trilinos 11.9 and greater), we do not run into some bad
code in Trilinos causing a segfault. At this point of the code, it is enough if we only
call FillComplete because there should be no communication after all.
Merge pull request #182 from kronbichler/master

Circumvent a bug in Trilinos 11.10
Provide better documentation on ghosted vectors.

This includes a discussion of what ghosted vectors are and what their
semantics are.