Easier debugging of neuron build cells

Hi Andrew,

Sorry to bother you with this pull request of minor benefit, but I didn't want it to get mixed in with other changes down the line just in case you had any stylistic complaints.

I have made some changes to my cell model class and was trying to debug its creation in the create_cells method, but ran into this frustrating recursive loop when trying to access the cells I had already created because the Population class wasn't properly initialised yet. After finally working out what was causing it, I was able to get around it by a couple of slight changes, which although they won't have any impact on the production code could make debugging of the cell creation in NEURON slightly easier in the future.

Cheers,

Tom