Implemented new spikespace

[thesamovar]

No description provided.

[coveralls]

Coverage remained the same when pulling 907675d on spikespace into 6ed4d2b on master.

[thesamovar]

Now that I know the Travis build passed I'm just going to go ahead and merge this in the interests of saving time.

[mstimberg]

Looks good to me and nicely simplifies the C++ code. I changed one small thing: _spikespace shouldn't be a non-constant AttributeVariable, it can be a simple ArrayVariable instead. One of the advantages of the _spikespace technique is that the reference does not change, therefore there is no need to execute getattr(group, '_spikespace') at every timestep and insert it into the namespace.

I guess we should get rid of the spikes attribute / Variable completely to make sure it is not used anywhere?

[thesamovar]

Nice! Yes, I left spikes because I didn't want to break any code but I guess it should go. Or maybe we want to keep it as it is, a property so that users can use it without needing to know about the spikespace implementation?