Fix the setting of "unless refractory" with string expressions that do not refer to the variable itself.
Yet again: allow an empty `code` argument for `CodeRunner`
Remove the `int(...)` from the string expressions in the barrelcortex example for now, see comments in #326, #337
Fix two regressions in standalone examples introduced during the OpenMP merge: N_incoming and N_outgoing were not updated during synapse creation and the wrong size argument was used in the "push" template when used with subgroups
Update `Clock.t` when running standalone code -- some Python code might expect it to change after a run, having always 0*ms stored there might be confusing.
Merge remote-tracking branch 'origin/master' into docs_improvements

Conflicts:
	brian2/groups/group.py
	examples/complex/kremer_et_al_2011_barrel_cortex.py