fix sign bug in matrixfree (introduced during py3 port)
fix sign bug in matrixfree (introduced during py3 port)

Yay regression tests!  I was trying to be clever (oops) and reduce
repeated code, but I missed the fact that these two options have
different signs (`-` for dirichlet, `+` for neumann).  Reverting back to
the old code here.

barbagroup/pygbe,barbagroup/pygbe,barbagroup/pygbe