Merge pull request #264 from brian-team/linked_variables

Linked variables
Use assert_allclose instead of assert_equals in test_state_variable_assignment to avoid test failures when weave with -ffast-math is used
Be a bit more careful about non-default index variables in `calc_indices`, they don't necessarily have the same length as the default `_idx` index.
First simple but working version of `SpikeGeneratorGroup`
Add a basic test for `SpikeGeneratorGroup`.
Allow to call `Nameable.__init__` without a name, for situations where a name has already been provided in a previous call.
Use the fact that spike times are sorted to more efficiently find spike times
Add `SpikeGeneratorGroup` template for C++ standalone (untested)
Add some notes to the `SpikeGeneratorGroup` docs
More thorough testing of `SpikeGeneratorGroup`, including test for standalone
Use the new "initialize arrays directly" mechanism in `SpikeGeneratorGroup`. Also sort times/indices not only by times but also by indices (if times are identical).