Add any missing error checks in linear algebra backends

From Bugzilla:

All linear algebra classes needs a thourough walk-through to make sure there
are checks for initialized vectors etc. everywhere it may cause a segmentation
fault.

------- Comment #1 From Martin Alnæs 2008-06-25 12:46:20 [reply] -------

The solution is to implement in-place operations as virtual functions, and
%extend GenericVector with python code for operators in the swig interface.

------- Comment #2 From Johan Hake 2008-10-01 13:09:42 [reply] -------

(In reply to comment #1)
> The solution is to implement in-place operations as virtual functions, and
> %extend GenericVector with python code for operators in the swig interface.

This part is now done. But I would suggest a more thorough walk through. Eg.
set(all) and get(all) can be used to seg fault the program.

Johan