poisson: verify sum of localized error contributions equals total estimated error

[bgranzow]

I've already done this, which is super great!
However I mostly hacked it into poisson_solver.cpp.
Maybe I could make this a little cleaner and add a
regression test that checks:
\sum_{i=1}^{n_{vtx}} \eta_i = z^T R(u_fine)

This also indicates that it might be useful to have the following library-wide APIs:
(1) sum all error contributions from physics->get_e()
(2) sum absolute values of error contributions from physics->get_e() ( an approximate upper bound )
I should open a new issue for this....

[bgranzow]

Split into two issues:
#40
#41