Working version of conditional_write with reset override

Summary of changes:
- override_conditional_write argument is added everywhere
- reset uses this argument
- refactored some conditional write code into base Language
- implemented weave version
- fixed bug in numpy version where it would only work for 1 neuron
(because newly created variables weren't handled correctly), this wasn't
an issue for the weave version
- created new test of the behaviour
- changed the standalone test, because the new refractoriness mechanism
changes the results